ci: replace manual cargo cache with Swatinem/rust-cache (#68)

Replace all 5 instances of manual actions/cache configuration with
Swatinem/rust-cache@v2 across build, lint, and test workflows.

Benefits:
- Smarter cache key (includes Cargo.lock + Cargo.toml + toolchain)
- Auto-cleanup of stale target artifacts to prevent cache bloat
- cache-on-failure saves cache even on build errors
- Simpler configuration (3 lines vs 10+ lines per job)

Author: panos-xyz

.github/workflows/build.yml

@@ -26,16 +26,10 @@ jobs:
         with:
           components: rustfmt, clippy
 
-      - name: Cache cargo registry
-        uses: actions/cache@v4
+      - name: Cache Rust build artifacts
+        uses: Swatinem/rust-cache@v2
         with:
-          path: |
-            ~/.cargo/registry
-            ~/.cargo/git
-            target
-          key: ${{ runner.os }}-cargo-build-${{ hashFiles('**/Cargo.lock') }}
-          restore-keys: |
-            ${{ runner.os }}-cargo-build-
+          cache-on-failure: true
 
       - name: Build debug
         run: cargo build --all --verbose

.github/workflows/lint.yml

@@ -40,16 +40,10 @@ jobs:
         with:
           components: clippy
 
-      - name: Cache cargo registry
-        uses: actions/cache@v4
+      - name: Cache Rust build artifacts
+        uses: Swatinem/rust-cache@v2
         with:
-          path: |
-            ~/.cargo/registry
-            ~/.cargo/git
-            target
-          key: ${{ runner.os }}-cargo-clippy-${{ hashFiles('**/Cargo.lock') }}
-          restore-keys: |
-            ${{ runner.os }}-cargo-clippy-
+          cache-on-failure: true
 
       - name: Run Clippy
         run: cargo clippy --all --all-targets -- -D warnings

.github/workflows/test.yml

@@ -23,16 +23,10 @@ jobs:
       - name: Install Rust toolchain
         uses: dtolnay/rust-toolchain@stable
 
-      - name: Cache cargo registry
-        uses: actions/cache@v4
+      - name: Cache Rust build artifacts
+        uses: Swatinem/rust-cache@v2
         with:
-          path: |
-            ~/.cargo/registry
-            ~/.cargo/git
-            target
-          key: ${{ runner.os }}-cargo-test-${{ hashFiles('**/Cargo.lock') }}
-          restore-keys: |
-            ${{ runner.os }}-cargo-test-
+          cache-on-failure: true
 
       - name: Run tests
         run: cargo test --all --verbose
@@ -47,16 +41,10 @@ jobs:
       - name: Install Rust toolchain
         uses: dtolnay/rust-toolchain@stable
 
-      - name: Cache cargo registry
-        uses: actions/cache@v4
+      - name: Cache Rust build artifacts
+        uses: Swatinem/rust-cache@v2
         with:
-          path: |
-            ~/.cargo/registry
-            ~/.cargo/git
-            target
-          key: ${{ runner.os }}-cargo-doc-${{ hashFiles('**/Cargo.lock') }}
-          restore-keys: |
-            ${{ runner.os }}-cargo-doc-
+          cache-on-failure: true
 
       - name: Run doc tests
         run: cargo test --doc --all --verbose