Add CI and release automation

.github/workflows/ci.yml

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+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+env:
+  CARGO_TERM_COLOR: always
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    name: Test (${{ matrix.build }})
+    runs-on: ${{ matrix.os }}
+    timeout-minutes: 20
+    strategy:
+      fail-fast: false
+      matrix:
+        build: [stable, beta, nightly]
+        include:
+          - build: stable
+            os: ubuntu-latest
+            rust: stable
+          - build: beta
+            os: ubuntu-latest
+            rust: beta
+          - build: nightly
+            os: ubuntu-latest
+            rust: nightly
+    steps:
+    - uses: actions/checkout@v6
+    - name: Install Rust
+      run: rustup update ${{ matrix.rust }} --no-self-update && rustup default ${{ matrix.rust }}
+      shell: bash
+    - run: cargo test
+    - run: cargo test --features serde
+
+  rustfmt:
+    name: Rustfmt
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+    - uses: actions/checkout@v6
+    - uses: dtolnay/rust-toolchain@stable
+      with:
+        components: rustfmt
+    - run: cargo fmt -- --check
+
+  semver-checks:
+    # TODO: Re-enable after initial crates.io publish
+    if: false
+    name: Semver Checks
+    runs-on: ubuntu-24.04
+    timeout-minutes: 10
+    steps:
+    - uses: actions/checkout@v6
+    - uses: obi1kenobi/cargo-semver-checks-action@v2
+
+  # Sentinel job for required checks - configure this job name in
+  # repository settings as the single required status check.
+  required-checks:
+    if: always()
+    needs: [test, rustfmt]
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - run: exit 1
+        if: >-
+          needs.test.result != 'success' ||
+          needs.rustfmt.result != 'success'

.github/workflows/cratesio-release.yml

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+# See https://crates.io/docs/trusted-publishing
+name: Publish to crates.io
+on:
+  push:
+    tags: ['v*']
+jobs:
+  publish:
+    runs-on: ubuntu-24.04
+    permissions:
+      id-token: write
+    steps:
+    - uses: actions/checkout@v6
+    - uses: rust-lang/crates-io-auth-action@v1
+      id: auth
+    - run: cargo publish
+      env:
+        CARGO_REGISTRY_TOKEN: ${{ steps.auth.outputs.token }}