[test] add CI workflow with Rust checks, cache setup, and auditing; update docs and README with CI details

.github/workflows/ci.yml

@@ -0,0 +1,45 @@
+name: CI
+
+on:
+  push:
+    branches: [ main, master ]
+  pull_request:
+    branches: [ main, master ]
+
+permissions:
+  contents: read
+
+jobs:
+  ci:
+    name: ci
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Setup Rust
+        uses: dtolnay/rust-toolchain@stable
+        with:
+          components: clippy, rustfmt
+      - name: Show rustc/cargo versions
+        run: |
+          rustc -Vv
+          cargo -Vv
+      - name: Cache cargo registry
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            target
+          key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
+      - name: Install cargo-audit
+        run: |
+          cargo install cargo-audit --locked || cargo install cargo-audit
+      - name: Format check
+        run: cargo fmt --all -- --check
+      - name: Clippy (warnings as errors)
+        run: cargo clippy --all-targets -- -D warnings
+      - name: Test
+        run: cargo test --all
+      - name: Audit
+        run: cargo audit