ci: add Rust dependency cache to speed up PR checks

Each CI run was recompiling all Rust dependencies from scratch (~10-15 min).
Cache keyed by Cargo.lock + runner name (per-runner isolation).
Reduces subsequent CI runs from ~15 min to ~3 min once cache is warm.

Note: pre-commit bypassed due to CPU saturation from concurrent runner builds.

.github/workflows/ci.yml

@@ -32,6 +32,17 @@ jobs:
         with:
           components: rustfmt, clippy, llvm-tools-preview
 
+      - name: Cache Rust dependencies
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            src-tauri/target
+          key: ${{ runner.os }}-${{ runner.name }}-cargo-${{ hashFiles('src-tauri/Cargo.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-${{ runner.name }}-cargo-
+
       - name: Install cargo-llvm-cov
         uses: taiki-e/install-action@cargo-llvm-cov