Read-only filtering happens in Surface.queueIo after callers construct
the message. This early return leaked write_alloc payloads because the
IO thread never receives them and therefore does not perform its normal
cleanup.
Read-only filtering happens in Surface.queueIo after callers construct
the message. This early return leaked write_alloc payloads because the
IO thread never receives them and therefore does not perform its normal
cleanup.
This removes use of global state from the crash reporting functionality
(everything in `src/crash`).
This particularly ensures that there are no races on the system
environment during the execution of the initialization thread that would
possibly cause crashes, particularly in any (albeit unsupported) 3rd
party integrations of libghostty-internal.
Ultimately, this pushes any coupling of I/O and environment to places
that would more correctly interface with global state, such as the
same-thread `global.init`, and the crash report CLI.
Note that similar de-coupling actions have been taken on XDG and home
directory functionality, pushing their coupling points up the stack in a
similar way.
Document why incremental history decoding exposes native page finalization
errors and intentionally bypasses the one-shot ExistingHistory guard after READY.
SCREEN decoding restored saved cursor coordinates directly from the wire
even when they exceeded the current terminal dimensions, unlike the live
cursor restoration path.
Lenient style decoding previously caught every error, so PAGE and SCREEN
could treat truncation or an I/O failure as an invalid semantic style and
continue from a corrupted stream position.
Add a nullable decoder that discards only invalid style contents while
propagating reader failures. Update snapshot callers and cover both semantic
fallback and structural failure behavior.
Decoded hyperlink table entries retained their insertion reference after
the grid added its per-cell references. Overwriting all linked cells could
therefore leave unused entries alive indefinitely.
Release each accepted wire table entry after grid decoding, including
duplicate values that map to one native ID. Regression coverage verifies
exact cell ownership and reaping after overwrite.
Grapheme suffix decoding accepted U+0000 even though zero is the native
empty-cell sentinel. It also appended codepoints one at a time and, when
page capacity failed after a prefix had been stored, left that truncated
prefix attached to the cell. Hostile snapshots could therefore introduce
invalid cluster data or render a partial cluster depending on allocator
capacity.
Ignore NUL alongside invalid scalar values. If any append runs out of
native capacity, remove the prefix already attached and consume the rest
of the declared suffix without applying it, making delivery atomic at the
cluster level. Cover NUL input and a failure after 128 accepted suffix
codepoints.
PAGE decoding inserted every valid style table entry into the native
ref-counted set before decoding cells. That insertion contributed one
reference in addition to every cell reference, unlike organically built
pages where the initial add belongs to the first cell. An unused encoded
style therefore remained live with refcount one and was emitted again on
every re-encode; used styles were also permanently over-counted.
After the grid has installed all cell references, release the temporary
table-owned reference once per distinct live style. Unused styles become
dead immediately and used styles retain exactly their cell count. Cover
used reference counts, unordered sparse IDs, and canonical first
re-encoding of an unused entry.
SCREEN encoding assumed every page from the active boundary onward was
resident. A debug assertion guarded that PageList policy invariant, but
release builds immediately used pageAssumeResident. If compression policy
ever allowed a SCREEN suffix page to remain compressed, the encoder would
read an inactive union field, causing undefined behavior and potentially
a crash or corrupt snapshot.
Use pagePreservingState for every SCREEN suffix page, as HISTORY already
does, and include allocation failure in EncodeError. Resident pages remain
a zero-allocation borrow while compressed pages decode into temporary
read-only storage without changing the source representation. Exercise the
path with an explicitly compressed active suffix page.
Decoder.next treated calls before READY and calls after any prior decode
error as unreachable. Network or mux glue that retried after a truncated
history record, or invoked next before setup completed, could therefore
turn a recoverable protocol misuse into a process panic.
Add DecoderNotReady and DecoderFailed to NextError and return them for
the start and failed states. Keep finished calls idempotent, and cover
both an early call and a retry after FINISH truncation.
SCREEN decode clamped the cursor x coordinate to the physical page
width, but validated pending wrap against the terminal-wide column
count. A lazily reflowed page narrower than the current terminal could
therefore lose a valid pending-wrap state at its last physical column.
The next write would continue on the same row instead of wrapping.
Validate pending wrap against the cursor page width, matching the clamp
and the page-local cursor pin. Add a mixed-width decode regression that
places the cursor at the narrow page boundary.
This removes use of global state from the crash reporting functionality
(everything in src/crash).
This particularly ensures that there are no races on the system
environment during the execution of the initialization thread that would
possibly cause crashes, particularly in any (albeit unsupported) 3rd
party integrations of libghostty-internal.
Ultimately, this pushes any coupling of I/O and environment to places
that would more correctly interface with global state, such as the
same-thread global.init, and the crash report CLI.
Note that similar de-coupling actions have been taken on XDG and home
directory functionality, pushing their coupling points up the stack in a
similar way.
This adds a new `terminal.snapshot.Decoder` that allows for incremental
decoding of a snapshot stream. There are two methods: `ready` builds up
the entire terminal up to READY. Then `next` acts like a Zig iterator
and applies incremental history as it becomes available. In between
calls to `ready` and `next` the caller can do whatever.
The use case for this: with a 1MB ascii stream, the time to decode to
READY is ~40us on my machine, versus 1.5ms for the entire history. This
means that a terminal could be rendered and visible after 40us rather
than waiting for the full terminal. This isn't a large terminal, but
that READY time should be pretty standard since screens don't get that
big, but history is unbounded.
This adds a new `terminal.snapshot.Decoder` that allows for incremental
decoding of a snapshot stream. There are two methods: `ready` builds up
the entire terminal up to READY. Then `next` acts like a Zig iterator
and applies incremental history as it becomes available. In between
calls to `ready` and `next` the caller can do whatever.
Reworks the terminal PAGE grid wire format and optimize both
encode/decode. Example improvements for 1MB of VT input w/ full
scrollback: ~30x smaller wire size, ~45x faster encoding and decoding.
> [!IMPORTANT]
>
> **Snapshot version 1 is still explicitly a work-in-progress format, so
this breaks wire compatibility**.
The original snapshot version I merged favored simplicity over
optimization. This was the format used a proof-of-concept in my own
projects, but I knew it wasn't what I wanted to ship. This PR looks at
the record formats and trades simplicity for performance, a fair trade
for a performance-sensitive binary format.
Overview of changes:
- **8-byte grid cells.** Cells are now one 64-bit word whose layout
deliberately coincides with the native cell. Previously, cells were 16
bytes each and in our 1MB corpus 97% of the data was `0`. Lol.
- **Blank trailing cells are not written.** Rows declare an encoded cell
count so trailing blank cells cost nothing.
- **Hardware CRC32C.** Added `src/crc32c.zig` that uses inline-asm on
aarch64/x86_64 to get hardware speeds for CRC32. Zig's stdlib is 0.56
GB/s, aarch64 hardware is 10 GB/s on my computer.
- **Variable-width cells.** Each row declares how many bytes transport
each cell word: 1, 2, 4, or 8 depending on the widest row cell.
## Format
Grid layout, per PAGE record:
```
old new
+--------------------------+ +--------------------------+
| row 0 | | row 0 |
| flags (1) | | flags + width (1) |
| cols * 16B cells with | | encoded cell count (2) |
| inline suffixes | | count * width cells |
+--------------------------+ +--------------------------+
| ... | | ... |
+--------------------------+ +--------------------------+
| row (rows - 1) | | row (rows - 1) |
+--------------------------+ +--------------------------+
| grapheme suffix section |
+--------------------------+
```
Every row previously carried exactly `cols` cells; now it carries cells
only through its last non-default cell, and the cells past the count are
implicitly zero. The row flag byte gains the encoded cell width in its
previously reserved bits:
```
bit 0 wrap bit 2-3 semantic prompt
bit 1 wrap continuation bit 4-5 encoded cell width (log2 bytes)
```
The cell itself, old fixed 16-byte header versus the new single word:
```
old (16 bytes + inline suffixes) new (one u64 word)
+--------+---------+--------+ bit 0 +------------------+
| kind 1 | width 1 | flags 1| | content kind 2b |
+--------+---------+--------+ bit 2 +------------------+
| zero 1 | style id 2 | | content 24b |
+--------+------------------+ bit 26 +------------------+
| hyperlink id 2 | | style ID 16b |
+---------------------------+ bit 42 +------------------+
| value 4 | | width kind 2b |
+---------------------------+ bit 44 +------------------+
| grapheme count 4 | | protected 1b |
+---------------------------+ bit 45 +------------------+
| grapheme cps 4 * count | | hyperlink 1b |
+---------------------------+ bit 46 +------------------+
| semantic 2b |
bit 48 +------------------+
| hyperlink ID 16b |
bit 64 +------------------+
```
The word's bit layout intentionally matches the native cell (with the
wire hyperlink ID in the native padding), so full-width rows are a
straight copy of page memory. The row's encoded width then transports
each word truncated, and decode is the matching zero-extension:
```
width | bytes | admitted cells
------+-------+------------------------------------------------
0 | 1 | codepoint <= U+00FF, nothing else set
1 | 2 | codepoint <= U+FFFF, nothing else set
2 | 4 | any content kind/codepoint, style IDs 1-63,
| | narrow, no flags, no hyperlink
3 | 8 | everything
```
Grapheme suffixes were inline after each cell, which forced per-cell
framing decisions; they are now one section after the rows, so a
grapheme-free page (the overwhelming case) pays 4 bytes total:
```
old: ... | cell | cp cp | cell | ... (inline, per cell)
new: +----------------+----------------------------------+
| entry count 4 | entries: row 2, col 2, count 2, |
| | count * codepoint 4 |
+----------------+----------------------------------+
```
## Performance
Setup: `ghostty-bench +terminal-snapshot`, 80x24 terminal with unlimited
scrollback fed 1 MB of VT input.
Per-commit improvements:
| change | wire size | encode | decode |
|-------------------------------|-----------|---------|----------|
| baseline (v1 before this PR) | 34.16 MB | 92.8 ms | 119.8 ms |
| 8-byte cells + blank elision | 7.66 MB | 18.2 ms | 28.0 ms |
| hardware CRC32C | 7.66 MB | 5.8 ms | 15.5 ms |
| gate page verification | 7.66 MB | 5.8 ms | 12.2 ms |
| staged PAGE payload decoding | 7.66 MB | 5.8 ms | 8.1 ms |
| variable-width cells | 1.03 MB | 2.0 ms | 2.7 ms |
Final result across various inputs:
| corpus | wire size | encode | decode |
|----------------------------|------------------------|----------------------|-----------------------|
| ascii lines 1-70 | 34.16 -> 1.03 MB (33x) | 92.8 -> 2.0 ms (46x) |
119.8 -> 2.7 ms (44x) |
| ascii full-width wrap | 16.01 -> 1.04 MB (15x) | 43.6 -> 1.3 ms (34x)
| 56.2 -> 1.8 ms (31x) |
| utf8 (wide/grapheme heavy) | 4.33 -> 1.89 MB (2.3x) | 12.4 -> 1.7 ms
(7x) | 19.0 -> 2.2 ms (9x) |
### Relationship with Compression
I expect that users of this will wrap everything in compression, so I
also benchmarked all my changes against a caller-owned zstd compressor
to ensure we're making the write tradeoffs. Less bytes means less time
in a compressor, even if a ton of 0s compresses really well.
My results: `zstd -1` over the `lines` snapshot drops from 12.7 ms to
0.8 ms, and the compressed artifact shrinks from 1.35 MB to 0.86 MB. So
the end state is a win-win.
Add a per-row encoded cell width to the PAGE grid format. Rows
previously always spent eight bytes per cell, but a plain text cell
carries only a codepoint: on line-shaped scrollback most encoded
bytes were predictable zeros that still had to pass through CRC32C,
BLAKE3, both codecs, and any transport compression the caller
applies.
Each row now declares one of four widths in previously reserved row
flag bits, chosen canonically as the smallest width admitted by the
bitwise OR of the row cell words: one or two bytes transport a bare
codepoint, four bytes transport the low word half (any content kind,
style IDs up to sixty-three, no wide or flag or hyperlink bits), and
eight bytes remain the full word. Every width is a truncation on
encode and a zero-extension on decode, so narrow rows encode and
decode as vectorizable integer loops, one and two byte rows need at
most surrogate replacement and skip cell normalization entirely, and
full-width rows keep the existing bulk copy. Decoders use the
declared width for framing and accept rows encoded wider than
necessary. Rows containing wide characters, hyperlinks, semantic
content, or large style IDs still use the full width, which leaves
CJK-heavy content unchanged.
Benchmark deltas at this commit (terminal-snapshot, M-series,
ReleaseFast, 1 MB corpora):
ascii lines 1-70: 7.66 MB -> 1.03 MB (7.4x)
encode 5.8 -> 2.0 ms, decode 8.1 -> 2.7 ms
ascii full-wrap: 8.04 MB -> 1.04 MB (7.7x)
encode 5.4 -> 1.3 ms, decode 7.2 -> 1.8 ms
utf8: unchanged (wide cells keep rows at full width)
For a caller compressing the stream, the lines snapshot end to end
with zstd -1: encode plus compress 18.5 -> 2.8 ms, decompress plus
decode 15.8 -> 3.6 ms, and the compressed size itself drops from
1.35 MB to 0.86 MB because the packed stream is denser for the
entropy coder.
PAGE payloads were decoded through a stack of stream adapters:
a CRC32C-hashing reader over a length-limited reader over the
BLAKE3-hashing snapshot reader. Every row paid several adapter
crossings and both hashes were fed row-sized chunks, which kept
BLAKE3 out of its efficient many-block path and made adapter
overhead about a quarter of decode time.
Decode now reads the remaining payload into a scratch buffer with
one bulk read, so each hash sees the payload as a single update, and
then parses the tables and grid from a flat in-memory reader. Row
headers are also read as one three-byte read instead of two calls.
Staging is capped at 8 MiB, far above any standard-capacity page
payload, so a hostile declared length cannot force a large
allocation; larger payloads fall back to the streaming path. CRC
validation and exact-exhaustion checks are unchanged, with the
staged reader checked for leftover bytes to preserve
PayloadNotExhausted semantics.
Benchmark deltas at this commit (terminal-snapshot, 1 MB corpora):
ascii lines 1-70: decode 12.2 -> 8.1 ms (encode unchanged)
ascii full-wrap: decode 11.1 -> 7.2 ms
utf8: decode 3.1 -> 2.1 ms
Relative to the previous wire format and codecs, the series is a
16.0x encode and 14.8x decode improvement on line-shaped scrollback
at 4.5x smaller wire size.
PAGE decoding verified the complete native integrity of every decoded
page unconditionally, building per-cell reference maps that accounted
for roughly a fifth of decode time. The decoder normalizes every
semantic value while decoding, so a completed decode upholds page
invariants by construction and the verification only defends against
decoder bugs. Follow the native page policy instead: assertIntegrity
and friends run full verification only when slow runtime safety is
enabled, which keeps the check in debug and test builds where those
bugs are caught.
Benchmark deltas at this commit (terminal-snapshot, 1 MB corpora):
ascii lines 1-70: decode 15.5 -> 12.2 ms (encode unchanged)
Rework the PAGE grid encoding for codec speed and size. This is a
breaking change to the work-in-progress version 1 wire format.
Cells were previously a fixed 16-byte header plus inline grapheme
suffixes: one byte each for content kind, width, and flags, a
reserved byte, 16-bit style and hyperlink IDs, a 32-bit value, and an
always-present 32-bit suffix count that was almost always zero. Cells
are now one 64-bit little-endian word with a documented bit registry
that carries the hyperlink ID in its high bits. The layout
deliberately coincides with the native cell so clean rows encode as a
straight copy of page memory and decode as one bulk read plus an
in-place normalization pass; a comptime check falls back to a
portable field-by-field codec if the native layout ever diverges.
Each row header also gains an encoded cell count so trailing default
cells are elided instead of spending 16 bytes apiece encoding
nothing: on typical shell output most of every row is blank, and
measurement showed 97% of encoded snapshot bytes were zero. Grapheme
suffixes move out of the cell stream into a per-grid section of
(row, column, codepoints) entries, which keeps row decoding
fixed-stride and bulk-copyable.
Decode ID remapping switches from hash maps to direct-indexed tables
sized by the 16-bit encoded ID space, removing per-styled-cell hash
lookups.
Benchmark deltas at this commit (terminal-snapshot, M-series,
ReleaseFast, 1 MB corpora):
ascii lines 1-70: 34.16 MB -> 7.66 MB (4.5x)
encode 92.8 -> 18.2 ms, decode 119.8 -> 28.0 ms
ascii full-wrap: 16.01 MB -> 8.04 MB (2.0x)
encode 43.6 -> 18.4 ms, decode 56.2 -> 25.6 ms
utf8: 4.33 MB -> 1.90 MB (2.3x)
encode 12.4 -> 4.9 ms, decode 19.0 -> 9.6 ms
Previously if you select "**Install and Relaunch**" in the update pill,
there's still a confirmation alert about killing active process, but it
will just relaunch regardlessly(which is the intended behaviour) when
you do it in the command palette.
Using UpdateState to check so Ghostty will relaunch immediately when
user chooses "Install and Relaunch".
> For the auto update case, this will be handled a bit differently in
the future. If an update is already installed and waiting for relaunch
(that the user is not aware of), quitting Ghostty will still remind them
if there's active processes.
OSC 52 clipboard writes decoded their base64 payload with the scalar
std implementation while Kitty graphics payloads already used the SIMD
decoder in src/simd.
Move clipboard path to use the same SIMD decoder. The encode side
of the read reply stays scalar since the codebase has no SIMD
encoder. 3.3x faster on a 4KB-payload decode micro-benchmark.
Measures the terminal binary snapshot codecs in both directions
against the same terminal state. Setup feeds a pre-generated VT
stream (for example from ghostty-gen ascii) to a terminal outside
the timed region.
Baseline measurements at this commit (M-series, ReleaseFast, 80x24,
unlimited scrollback, 1 MB corpora, per-loop time with setup
subtracted):
ascii lines 1-70: 34.16 MB encode 92.8 ms decode 119.8 ms
ascii full-wrap: 16.01 MB encode 43.6 ms decode 56.2 ms
utf8: 4.33 MB encode 12.4 ms decode 19.0 ms
The ascii generator emits an unbroken stream of printable bytes, which
exercises terminal wrapping but produces only full-width rows. Add
line-min and line-max options that emit CR LF-terminated lines with a
uniformly distributed printable length so generated corpora can also
model shell-like output where most rows end well before the last
column. The default behavior is unchanged.
In the html formatter every page is formatted in a div. When the div
closes it causes a newline in the html rendering. In order to fix this a
newline is now removed whenever the div is closed (if there are any
newlines waiting to be rendered - as far as I could see in my testing
there was always one).
I thought of trying to add a test but could not think of a way to do so
without adding a massive blob of html into the file.
Before (orange was added by me to show where the div ends):
<img width="1278" height="586" alt="image"
src="https://github.com/user-attachments/assets/473ba28d-bec0-481f-9a89-a6a72c9a3657"
/>
After:
<img width="959" height="439" alt="image"
src="https://github.com/user-attachments/assets/27bae0d3-cc82-4dc8-aa72-c4a8b0f7d424"
/>
No AI was used in this pr.
### What
Comment-only cleanup in `src/termio/message.zig`:
- Fixes a subject-verb agreement error in the `Message` union's doc
comment: "the number of messages ... are also very few" -> "is also
very small"
- Capitalizes "pty" to "PTY" in several doc comments
### Why
Caught while reading through `message.zig`. No functional changes,
doc comments only.
Builds on #13544
This adds a new CONTINUATION record type that is sent before READY.
CONTINUATION contains the bytes (if any) that will bring a ground-state
VT state machine up to the same state.
This allows snapshotting a terminal instance that is, for example,
blocked waiting for a caller to complet an in-flight Kitty graphics
protocol send. In practice, I think this will be rare. But in theory, it
avoids a DoS-type attack.
The continuation state must be the MINIMAL set of bytes that will move
the virtual terminal state from a ground to non-ground state. The reason
it must be minimal is because any extra bytes can duplicate work into
the terminal that might already exist.
Builds on #13544
This adds a new CONTINUATION record type that is sent before READY.
CONTINUATION contains the bytes (if any) that will bring a ground-state
VT state machine up to the same state.
This allows snapshotting a terminal instance that is, for example,
blocked waiting for a caller to complet an in-flight Kitty graphics
protocol send. In practice, I think this will be rare. But in theory, it
avoids a DoS-type attack.
The continuation state must be the MINIMAL set of bytes that will move
the virtual terminal state from a ground to non-ground state. The reason
it must be minimal is because any extra bytes can duplicate work into the
terminal that might already exist.
- "the number of messages we send to the IO thread are also very few"
had a subject-verb agreement issue; reworded to "is also very small"
- Capitalized "pty" -> "PTY"
This adds opt-in continuation tracking to `terminal.Stream` that allows
any caller to call `writeContinuation` in order to get the minimum bytes
necessary from a grounded parser state to the identical state.
This enables reliable stream restart across serialization states, which
could be used for local restart, networked terminals, etc. For me, this
is used for multiplexers. :)
**LLM usage:** I wrote the continuation tracker myself, used a mix of
5.6+Fable to review it for me, applied their feedback directly. Only
place with predominantly AI code are tests, which I reviewed. Commit and
PR message written myself.
## Implementation
The implementation of this was really carefully done to avoid any
negative performance impact particularly when continuation tracking is
_off_.
The way this work is simple:
1. ESC is the only char that leaves the ground state and most ESC
sequences are short. So if we're in a non-ground state, we do a
backwards vectorized search to find the last `ESC` in the input slice.
If one doesn't exist, we assume we found it previously and store the
whole slice (rare, since ESC sequences are usually short like I said).
2. If we're in the ground state that means we only have a potential
incomplete UTF-8 codepoint, so we find the lead UTF-8 byte.
3. When writing, we normalize the suffix to drop things like BEL
commands that would've already been handled to avoid double-calling.
## Performance
No real impact.
Via `ghostty-bench +terminal-stream`.
Corpus | Main | PR w/ Tracking Off | PR w/ Tracking On
-- | -- | -- | --
Plain ASCII (256 MiB) | 175.4 ms | 175.8 ms | 175.5 ms
UTF-8 (32 MiB) | 268.4 ms | 270.0 ms | 269.9 ms
5% invalid UTF-8 (32 MiB) | 316.4 ms | 317.8 ms | 320.2 ms
CSI-heavy (32 MiB) | 145.0 ms | 146.3 ms | 145.9 ms
OSC (32 MiB) | 1621.0 ms | 1627.5 ms | 1638.3 ms
Kitty APC (128 MiB) | 95.5 ms | 96.9 ms | 96.3 ms
Mixed traffic (32 MiB) | 172.4 ms | 172.0 ms | 172.3 ms
Giant APC (128 MiB) | 38.3 ms | 38.3 ms | 40.8 ms
This adds opt-in continuation tracking to `terminal.Stream` that allows
any caller to call `writeContinuation` in order to get the minimum bytes
necessary from a grounded parser state to the identical state.
This enables reliable stream restart across serialization states, which
could be used for local restart, networked terminals, etc. For me, this
is used for multiplexers. :)
## Implementation
The implementation of this was really carefully done to avoid any
negative performance impact particularly when continuation tracking is
_off_.
The way this work is simple:
1. ESC is the only char that leaves the ground state and most
ESC sequences are short. So if we're in a non-ground state, we
do a backwards vectorized search to find the last `ESC` in the
input slice. If one doesn't exist, we assume we found it previously
and store the whole slice (rare, since ESC sequences are usually
short like I said).
2. If we're in the ground state that means we only have a potential
incomplete UTF-8 codepoint, so we find the lead UTF-8 byte.
3. When writing, we normalize the suffix to drop things like BEL
commands that would've already been handled to avoid
double-calling.
## Performance
Via `ghostty-bench +terminal-stream`
Corpus main tracking off tracking on
plain ASCII (256 MiB) 175.4ms 175.8ms 175.5ms
UTF-8 (32 MiB) 268.4ms 270.0ms 269.9ms
5% invalid UTF-8 (32 MiB) 316.4ms 317.8ms 320.2ms
CSI-heavy (32 MiB) 145.0ms 146.3ms 145.9ms
OSC (32 MiB) 1621.0ms 1627.5ms 1638.3ms
Kitty APC (128 MiB) 95.5ms 96.9ms 96.3ms
mixed traffic (32 MiB) 172.4ms 172.0ms 172.3ms
giant APC (128 MiB) 38.3ms 38.3ms 40.8ms
Every page is formatted in a div, and when the div closes it creates a newline in the html rendering.
In order to fix this a newline is now removed whenever the div is closed (if there are any newlines waiting to be rendered).
## Summary
- normalize circular-buffer metadata after every resize
- retain the oldest values when shrinking below the current length
- cover partial shrink, exact-length shrink, and empty-to-zero
boundaries
## Root cause
`resize` rotated live values to index zero before reallocating, but only
repaired `head` and `full` when capacity grew. Shrinking a partially
filled buffer could therefore leave `head` beyond the new allocation and
report a length greater than capacity. A later append could index
outside the resized storage.
## Validation
- `zig fmt --check src/datastruct/circ_buf.zig`
- `zig test src/circ_buf_test.zig --test-filter 'CircBuf resize'` using
a temporary import harness: 8 tests passed
