terminal/snapshot: remove BLAKE3 digests

Remove BLAKE3 prefix digests. Keep READY/FINISH as empty records since
they're semantically important markers.

Our existing format (CRC32 per-record, declared counts, strict tag ordering
requirements, etc.) already detect: accidental corruption, truncation,
data omission, and duplication. 

BLAKE3 only protects against valid records being swapped or removed entirely. 
It is heavy for just that, and callers can solve that anyways via their
own transport (like, just use TCP). For more adversarial protection,
callers can also add layers like TLS or their own alternate signing
methods depending on their own threat models.

Removing the hash improves encode times by ~1.4x, decode times by ~1.3x.
Time-to-READY decoding is effectively unchanged because it was such a 
small package to begin with.

include/ghostty/vt/snapshot.h

@@ -23,12 +23,12 @@ extern "C" {
  *
  * Encode and restore the complete state of a terminal via a binary format.
  *
- * A snapshot is an ordered, authenticated record stream. Its READY checkpoint
- * contains enough state to render and resume the terminal, including any
+ * A snapshot is an ordered, CRC-protected record stream. Its READY marker
+ * follows enough state to render and resume the terminal, including any
  * unfinished VT parser input. Older scrollback pages follow READY and the
- * FINISH checkpoint authenticates the complete snapshot.
+ * FINISH marker terminates the complete snapshot.
  *
- * End-of-file before an operation's required READY or FINISH checkpoint is
+ * End-of-file before an operation's required READY or FINISH marker is
  * malformed, truncated snapshot data and returns GHOSTTY_INVALID_VALUE.
  * GHOSTTY_IO_ERROR is reserved for a reader callback that returns false.
  *
@@ -91,22 +91,22 @@ extern "C" {
  * +------------- CONTINUATION ---------------+
  * | unfinished VT/UTF-8 input, or ground     |
  * +------------------ READY -----------------+
- * | BLAKE3-256 of every preceding byte       |  ready() returns here
+ * | empty renderable-state marker            |  ready() returns here
  * +----------------- HISTORY ----------------+  repeated per screen
  * | scrollback manifest                      |
  * +------------------ PAGE ------------------+  next() consumes one page
  * | older screen rows                        |
  * +------------------ FINISH ----------------+
- * | BLAKE3-256 of every preceding byte       |  next() returns NO_VALUE
+ * | empty end-of-snapshot marker              |  next() returns NO_VALUE
  * +------------------------------------------+
  * | trailing transport bytes (not consumed) |
  * +------------------------------------------+
  * @endcode
  *
- * READY authenticates the renderable prefix through CONTINUATION. FINISH
- * authenticates READY and every history record as well as the earlier prefix.
- * Thus record CRC32C detects local corruption while the BLAKE3 checkpoints
- * also bind the ordering and completeness of the record stream.
+ * READY separates the renderable prefix through CONTINUATION from history.
+ * FINISH terminates the record sequence. Both are empty records protected by
+ * CRC32C, like every other record. Declared record counts, tags, and strict
+ * decoding enforce the stream's ordering and completeness.
  *
  * Snapshot format version 1 is a work in progress and does not yet carry a
  * binary-compatibility guarantee.
@@ -206,7 +206,7 @@ typedef enum GHOSTTY_ENUM_TYPED {
   /**
    * Rows prepended by the most recently decoded history page.
    *
-   * Zero means the page was consumed and authenticated but could not be
+   * Zero means the page was consumed and validated but could not be
    * applied to the live terminal.
    *
    * Output type: size_t *
@@ -238,7 +238,7 @@ typedef enum GHOSTTY_ENUM_TYPED {
  * otherwise this returns GHOSTTY_INVALID_VALUE.
  *
  * Encoding begins at the writer's current position. If an error occurs, the
- * writer may contain a partial snapshot without a valid FINISH checkpoint.
+ * writer may contain a partial snapshot without a valid FINISH marker.
  * Calls to the writer are synchronous; this function does not flush or make
  * the caller's destination durable.
  *
@@ -319,7 +319,7 @@ GHOSTTY_API GhosttyResult ghostty_snapshot_encode_alloc(
  * wait outside the decoder or block in their callback. The read callback must
  * not call APIs, including ghostty_snapshot_decoder_free(), on the decoder
  * that owns it. Returning false reports GHOSTTY_IO_ERROR; returning true with
- * zero bytes before a required checkpoint reports truncated snapshot data as
+ * zero bytes before a required marker reports truncated snapshot data as
  * GHOSTTY_INVALID_VALUE.
  *
  * @param allocator Allocator for decoder and decoded terminal state, or NULL
@@ -390,7 +390,7 @@ GHOSTTY_API GhosttyResult ghostty_snapshot_decoder_set(
     const void* value);
 
 /**
- * Decode and authenticate the renderable snapshot prefix through READY.
+ * Decode and validate the renderable snapshot prefix through READY.
  *
  * On success, terminal receives a caller-owned terminal with its persistent
  * VT stream already restored from the snapshot continuation. The terminal is
@@ -425,14 +425,14 @@ GHOSTTY_API GhosttyResult ghostty_snapshot_decoder_ready(
 /**
  * Decode one history page into the terminal returned by READY.
  *
- * Each GHOSTTY_SUCCESS consumes and authenticates one PAGE record. Query the
+ * Each GHOSTTY_SUCCESS consumes and validates one PAGE record. Query the
  * GHOSTTY_SNAPSHOT_DECODER_DATA_PROGRESS_* values before calling next again.
  * GHOSTTY_NO_VALUE means FINISH was validated; repeated calls after FINISH
  * also return GHOSTTY_NO_VALUE.
  *
  * The terminal may be rendered, resized, and fed live PTY input between calls.
  * If a history page can no longer be applied safely, it is still consumed and
- * authenticated and progress reports zero rows. The decoder applies history
+ * validated and progress reports zero rows. The decoder applies history
  * to the caller-owned terminal produced by its READY operation.
  *
  * A decoding error invalidates the decoder's source position. The terminal
@@ -449,7 +449,7 @@ GHOSTTY_API GhosttyResult ghostty_snapshot_decoder_next(
     GhosttySnapshotDecoder decoder);
 
 /**
- * Decode and authenticate one complete snapshot.
+ * Decode and validate one complete snapshot.
  *
  * This is the one-shot form of READY followed by all history pages through
  * FINISH. It may only be called before decoding starts. Bytes following FINISH