Update to Zig 0.16.0 (#12726)

Closes #12228
Supersedes #12388

**UPDATED** - Also check comments for additional details!

This commit represents the majority of the work necessary to upgrade
Ghostty to use Zig 0.16.0.

At this point, all tests pass under Linux, but more work may be
necessary to get them to build and function on other platforms.

There are some parts of this update that deserve commentary, so that
follows below:

## Expanded use of global state (IO/environment related)

Global state, once generally only used by the C library, has now been
expanded to be used across the project at large. The static local
variable that holds the state has been moved private in its source
container with all attributes that need to be accessed globally gated
behind accessors, most of which guard on testing and send test copies
instead. Use of the global state in non-testing scenarios asserts that
the state has been initialized through `init` naturally through the
optional assertion process.

The rationale for this change is to have a location to store a
general-purpose I/O implementation and environment variables, both of
which are now provided through [Juicy
Main](https://ziglang.org/download/0.16.0/release-notes.html#Juicy-Main)
and hence can no longer be accessed or mutated through stdlib without
use of lower-level system calls and hacks (some of which are employed,
but sparingly).

As the code matures, dependence on global state should naturally slim
down.

We do not allow global state to be used in libghostty-vt. There are
comptime guards that prevent this should compilation of libghostty-vt
end up pulling `global.zig`. This means that as per the last paragraph,
work has already begun to de-couple the codebase from global state where
necessary. Additionally, in some places where environment needs to be
updated and where it can be done in an isolated fashion, environment
maps are used - system-level injection of environment through the use of
`setenv` or `unsetenv` now only happens during early initialization (and
hopefully we can remove these in the future too, especially since they
require re-synchronization of the higher-level environment primitives
after this is done).

## The `lib/compat` Tree

Some stdlib features that have been removed but still either seem they
would be valuable to us or outright complex to move away from
(particularly `SegmentedList`) have been extracted from 0.15.2, updated
as needed, and placed in `src/lib/compat`. The intention again is to
allow for piecemeal migration to more modern implementations or possibly
straight local versions.

This paradigm has also allowed us to add `std.Io.Condition.waitTimeout`,
which incidentally was missed in the 0.16.0 shuffle and has been
re-added for 0.17.0. We can remove this in favor of the upstream when we
eventually migrate to that, obviously.

Note that there was a lot more of this extracted code when this work was
started, but a lot of said code has been removed (namely environment or
process/fd-related functionality).

## translate-c Issues (functional on Linux, Darwin WIP)

There have been a number of C translation issues that we have been
working through through submitted patches and the great help from folks
on the Arocc and Zig side. This is ongoing, with the remaining work to
getting things fixed mainly focused on the MacOS side. Stay tuned for
further developments.

As mentioned at the top, follow comments for more details!

.github/workflows/release-tag.yml

@@ -153,7 +153,7 @@ jobs:
           authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
 
       - name: XCode Select
-        run: sudo xcode-select -s /Applications/Xcode_26.3.app
+        run: sudo xcode-select -s /Applications/Xcode_26.6.app
 
       - name: Xcode Version
         run: xcodebuild -version

.github/workflows/release-tip.yml

@@ -362,7 +362,7 @@ jobs:
           authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
 
       - name: Xcode Select
-        run: sudo xcode-select -s /Applications/Xcode_26.3.app
+        run: sudo xcode-select -s /Applications/Xcode_26.6.app
 
       - name: Build XCFramework
         run: nix develop -c zig build -Demit-lib-vt -Doptimize=ReleaseFast
@@ -454,7 +454,7 @@ jobs:
           authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
 
       - name: XCode Select
-        run: sudo xcode-select -s /Applications/Xcode_26.3.app
+        run: sudo xcode-select -s /Applications/Xcode_26.6.app
 
       - name: Xcode Version
         run: xcodebuild -version
@@ -710,7 +710,7 @@ jobs:
           authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
 
       - name: XCode Select
-        run: sudo xcode-select -s /Applications/Xcode_26.3.app
+        run: sudo xcode-select -s /Applications/Xcode_26.6.app
 
       - name: Xcode Version
         run: xcodebuild -version
@@ -907,7 +907,7 @@ jobs:
           authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
 
       - name: XCode Select
-        run: sudo xcode-select -s /Applications/Xcode_26.3.app
+        run: sudo xcode-select -s /Applications/Xcode_26.6.app
 
       - name: Xcode Version
         run: xcodebuild -version

.github/workflows/test.yml

@@ -335,7 +335,7 @@ jobs:
           authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
 
       - name: Xcode Select
-        run: sudo xcode-select -s /Applications/Xcode_26.3.app
+        run: sudo xcode-select -s /Applications/Xcode_26.6.app
 
       - name: Build XCFramework
         run: nix develop -c zig build -Demit-lib-vt
@@ -645,7 +645,7 @@ jobs:
           authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
 
       - name: Xcode Select
-        run: sudo xcode-select -s /Applications/Xcode_26.3.app
+        run: sudo xcode-select -s /Applications/Xcode_26.6.app
 
       - name: Build
         run: |
@@ -1077,7 +1077,7 @@ jobs:
           authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
 
       - name: Xcode Select
-        run: sudo xcode-select -s /Applications/Xcode_26.3.app
+        run: sudo xcode-select -s /Applications/Xcode_26.6.app
 
       - name: Xcode Version
         run: xcodebuild -version
@@ -1137,7 +1137,7 @@ jobs:
           authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
 
       - name: Xcode Select
-        run: sudo xcode-select -s /Applications/Xcode_26.3.app
+        run: sudo xcode-select -s /Applications/Xcode_26.6.app
 
       - name: Xcode Version
         run: xcodebuild -version
@@ -1196,9 +1196,29 @@ jobs:
       - name: Test GTK Build
         run: nix develop -c zig build -Dapp-runtime=gtk -Demit-docs -Demit-webdata
 
-      # This relies on the cache being populated by the commands above.
+        # NOTE: This used to point to the Zig global cache directory before
+        # 0.16.0. Now, the cache directory exclusively stores "minified"
+        # archives of each dependency (i.e., only the paths defined in
+        # build.zig.zon), and an uncompressed, working copy is stored in
+        # PROJECT_ROOT/zig-pkg. The zig-pkg directory is warmed up during the
+        # regular fetching process from the global cache directory if
+        # possible.
+        #
+        # Nothing else should change; zon2nix's store path mimics what a
+        # zig-pkg directory should look like, so actual build steps should be
+        # fine, it's just here where we assume that all dependencies have been
+        # downloaded, and we want to test the validity of the package
+        # directory, do we need to have it pointing to zig-pkg and not the
+        # zon2nix store path.
+        #
+        # Note that this can also be attempted by running:
+        #   zig build --fetch=needed
+        #
+        # Note that --system PKGDIR should not be added to this invocation as
+        # it disables fetching altogether (even, seemingly, from the local
+        # cache).
       - name: Test System Build
-        run: nix develop -c zig build --system ${ZIG_GLOBAL_CACHE_DIR}/p
+        run: nix develop -c zig build --system "$(realpath zig-pkg)"
 
   test-lib-vt:
     if: github.repository == 'ghostty-org/ghostty' && needs.skip.outputs.skip != 'true'
@@ -1377,7 +1397,7 @@ jobs:
           authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
 
       - name: Xcode Select
-        run: sudo xcode-select -s /Applications/Xcode_26.3.app
+        run: sudo xcode-select -s /Applications/Xcode_26.6.app
 
       - name: Xcode Version
         run: xcodebuild -version

.github/workflows/vouch-check-issue.yml

@@ -18,7 +18,7 @@ jobs:
         with:
           sparse-checkout: .github/issue-unvouched-message
 
-      - uses: mitchellh/vouch/action/check-issue@d66fa29a64600490892131ad87597c30c91fcac4 # unreleased main
+      - uses: mitchellh/vouch/action/check-issue@d66fa29a64600490892131ad87597c30c91fcac4 # v1.5.0
         with:
           issue-number: ${{ github.event.issue.number }}
           auto-close: true