Optimize CI for self-hosted runners and make macOS tests on-demand only

Remove rust-toolchain/rust-cache setup (pre-installed on self-hosted),
disable checkout clean, enable sccache and incremental compilation.
Split macOS tests into a separate job that only runs on workflow_dispatch
since release.yml already gates on macOS tests.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/rust.yml

@@ -39,32 +39,45 @@ jobs:
       - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
         with:
           persist-credentials: false
-      - uses: dtolnay/rust-toolchain@631a55b12751854ce901bb631d5902ceb48146f7 # stable
-        with:
-          components: rustfmt
+          clean: false
       - run: cargo fmt --check --all
 
   clippy:
     name: Clippy
     runs-on: self-hosted
+    env:
+      RUSTC_WRAPPER: sccache
+      SCCACHE_DIR: /home/bhelmkamp/.cache/sccache
+      CARGO_INCREMENTAL: "1"
     steps:
       - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
         with:
           persist-credentials: false
-      - uses: dtolnay/rust-toolchain@631a55b12751854ce901bb631d5902ceb48146f7 # stable
-        with:
-          components: clippy
-      - uses: Swatinem/rust-cache@779680da715d629ac1d338a641029a2f4372abb5 # v2
-        with:
-          cache-on-failure: true
+          clean: false
       - run: cargo clippy --workspace -- -D warnings
+      - name: Show sccache stats
+        run: sccache --show-stats
 
   test:
-    name: Test (${{ matrix.os }})
-    runs-on: ${{ matrix.os }}
-    strategy:
-      matrix:
-        os: [self-hosted, macos-15]
+    name: Test (Linux)
+    runs-on: self-hosted
+    env:
+      RUSTC_WRAPPER: sccache
+      SCCACHE_DIR: /home/bhelmkamp/.cache/sccache
+      CARGO_INCREMENTAL: "1"
+    steps:
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
+        with:
+          persist-credentials: false
+          clean: false
+      - run: cargo test --workspace
+      - name: Show sccache stats
+        run: sccache --show-stats
+
+  test-macos:
+    name: Test (macOS)
+    if: github.event_name == 'workflow_dispatch'
+    runs-on: macos-15
     steps:
       - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
         with: