ci: add GitHub Actions workflow (rustfmt + nextest) (#2)

Based on tidu-rs CI. Runs formatting check and tests on ubuntu + macos.

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

Author: shinaoka

.github/workflows/ci.yml

@@ -0,0 +1,47 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+env:
+  CARGO_TERM_COLOR: always
+
+jobs:
+  rustfmt:
+    name: rustfmt
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          components: rustfmt
+      - uses: Swatinem/rust-cache@v2
+      - run: cargo fmt --all --check
+
+  nextest:
+    name: nextest (${{ matrix.os }})
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, macos-latest]
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+      - uses: Swatinem/rust-cache@v2
+      - uses: taiki-e/install-action@nextest
+
+      - name: Run tests with nextest
+        run: cargo nextest run --workspace --release --no-fail-fast
+
+      - name: Run doctests
+        run: cargo test --doc --workspace --release