Update to Zig 0.16.0

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

Key parts:

* In addition to its previous responsibilities, the global state now
  houses state for global I/O implementations and the process
  environment. It is now also utilized in the main application along
  with the C library. Where necessary, global state is isolated from key
  parts of the implementation (e.g., in libghostty subsystems), and it's
  expected that this list will grow.

* We currently manage our own C translation layer where necessary. In
  these cases, cImport has been removed in favor of the new external
  translate-c package. Due to fixes that have needed be made to properly
  translate the dependencies that were swapped out, as mentioned, we
  have had to backport fixes from the current translate-c package (and
  the upstream Arocc dependency). We will host this ourselves until Zig
  0.17.0 is released with these fixes.

* Where necessary (only a small number of cases), some stdlib code from
  0.15.2 (and even from 0.17.0) has been taken, adopted, and vendored in
  lib/compat.

Co-authored-by: Leah Amelia Chen <hi@pluie.me>

.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