fix(ci): bust stale Homebrew glib cache on macOS runners

The Swatinem/rust-cache action was restored before brew installed the
native dependencies. When Homebrew updated glib (2.88.0 → newer) the
cached Rust build artifacts still referenced the old versioned Cellar
path (/opt/homebrew/Cellar/glib/2.88.0/lib), causing a linker failure:

    ld: library 'gio-2.0' not found

Fix by:
1. Moving `brew install` before the rust-cache step so libs are current
   before the cache is consulted.
2. Capturing `brew list --versions glib gtk4 libadwaita` into a
   prefix-key so the cache key changes whenever those packages update,
   forcing a clean Rust build with fresh pkg-config paths.

Linux and Windows cache behaviour is unchanged (MACOS_LIB_VER unset →
empty prefix-key → same default v0- prefix).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/rust.yml

@@ -40,7 +40,6 @@ jobs:
           - test
     steps:
       - uses: actions/checkout@v6
-      - uses: Swatinem/rust-cache@v2
       - name: Install Linux deps
         if: runner.os == 'Linux'
         run: |
@@ -49,6 +48,13 @@ jobs:
       - name: Install macOS dependencies
         if: runner.os == 'macOS'
         run: brew install gtk4 libadwaita imagemagick
+      - name: Record macOS native lib versions for cache key
+        if: runner.os == 'macOS'
+        run: |
+          echo "MACOS_LIB_VER=$(brew list --versions glib gtk4 libadwaita | tr '\n' '_')" >> "$GITHUB_ENV"
+      - uses: Swatinem/rust-cache@v2
+        with:
+          prefix-key: ${{ env.MACOS_LIB_VER }}
       - name: Install Windows Dependencies - create gtk dir
         if: runner.os == 'Windows'
         run: mkdir C:\gtk-build\gtk\x64\release