ci: split into fast check job and full test job

- check job (push + PR): fmt + clippy only, fast syntax validation
- test job (PR only): fmt + clippy + tests + docs, full validation

.github/workflows/ci.yml

@@ -9,7 +9,19 @@ env:
   CARGO_TERM_COLOR: always
 
 jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: Swatinem/rust-cache@v2
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          components: rustfmt, clippy
+      - run: cargo fmt --check
+      - run: cargo clippy --all-features --all-targets -- -D warnings
+
   test:
+    if: github.event_name == 'pull_request'
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4