ci: split clippy and tests for parallel execution

Split the combined rust-checks job into separate clippy and tests jobs
to maximise parallelisation and reduce overall CI time.

Previous performance:
- Combined clippy + tests in single job: ~5.5 minutes
- Running sequentially wasted runner capacity

New approach:
- Three fully parallel jobs: fast-checks, clippy, and tests
- No dependencies between jobs for maximum parallelisation
- Each heavy job runs independently: ~4.5 minutes each
- Better cache utilisation with shared Rust registry and system packages
- Overall pipeline completes faster with true parallelisation

Optimisations:
- Remove sccache from clippy job (doesn't compile, only analyses)
- Keep sccache for tests job where compilation caching provides benefit
- Add concurrency group to cancel in-progress runs on new pushes
- Remove job dependencies to enable full parallelisation

The duplication of setup steps (uv, rustup, system deps) is offset by
the parallelisation gains, as runners can process all checks simultaneously.

.forgejo/workflows/rust-checks.yml

@@ -4,6 +4,11 @@ on:
   push:
   pull_request:
 
+# Cancel in-progress runs when a new push is made to the same branch
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   fast-checks:
     name: Prek & Format
@@ -34,10 +39,61 @@ jobs:
         run: |
           cargo +nightly fmt --all -- --check
 
-  rust-checks:
-    name: Clippy & Tests
+  clippy:
+    name: Clippy
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          persist-credentials: false
+
+      - name: Install uv
+        uses: https://github.com/astral-sh/setup-uv@v6
+        with:
+          enable-cache: true
+          ignore-nothing-to-cache: true
+          cache-dependency-glob: ''  # Disable Python dependency tracking for Rust project
+
+      - name: Install Rust toolchain
+        run: |
+          # Install toolchain from rust-toolchain.toml
+          uvx rustup show  # This will auto-install from rust-toolchain.toml
+
+      - name: Install system dependencies
+        run: sudo apt-get update
+
+      - name: Cache system packages
+        uses: https://github.com/awalsh128/cache-apt-pkgs-action@v1
+        with:
+          packages: clang liburing-dev
+          version: 1
+
+      - name: Cache Rust registry
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/git
+            !~/.cargo/git/checkouts
+            ~/.cargo/registry
+            !~/.cargo/registry/src
+          key: rust-registry-${{hashFiles('**/Cargo.lock') }}
+
+      - name: Run Clippy lints
+        run: |
+          cargo clippy \
+            --workspace \
+            --features full \
+            --locked \
+            --no-deps \
+            --profile test \
+            -- \
+            -D warnings
+
+  tests:
+    name: Tests
     runs-on: ubuntu-latest
-    needs: fast-checks
     env:
       SCCACHE_ENABLED: ${{ vars.GH_APP_ID != '' && secrets.GH_APP_PRIVATE_KEY != '' }}
 
@@ -101,16 +157,6 @@ jobs:
         with:
           key: sccache-v0
           path: .
-      - name: Run Clippy lints
-        run: |
-          cargo clippy \
-            --workspace \
-            --features full \
-            --locked \
-            --no-deps \
-            --profile test \
-            -- \
-            -D warnings
 
       - name: Run Cargo tests
         run: |