ci: GitHub Actions workflow + drop test-fuzz-long

Eight parallel jobs on push/PR:

- test × 3 (default / no-default / all-features) via nextest + doc
- clippy × 3 feature permutations, -D warnings
- fmt
- doc (all-features, -D warnings on rustdoc)
- deny (cargo-deny action)
- miri (nightly, lib + vectors under default + no-default)
- fuzz (nightly, 60 s smoke on every target)
- mutants × 2 feature configs (same equivalence exclusions as local)

Swatinem/rust-cache on every cargo job; taiki-e/install-action for
nextest / cargo-fuzz / cargo-mutants. Uploads mutants.out on failure.

Drops test-fuzz-long from the justfile — 60 s per target in CI every
push plus the local proptest 1M-case suite already covers this.

.github/workflows/ci.yml

@@ -0,0 +1,161 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+env:
+  CARGO_TERM_COLOR: always
+  RUST_BACKTRACE: 1
+
+jobs:
+  test:
+    name: Test (${{ matrix.features }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        features:
+          - default
+          - no-default-features
+          - all-features
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+      - uses: Swatinem/rust-cache@v2
+      - uses: taiki-e/install-action@v2
+        with:
+          tool: nextest
+      - name: cargo nextest run
+        run: |
+          case "${{ matrix.features }}" in
+            default)             cargo nextest run ;;
+            no-default-features) cargo nextest run --no-default-features ;;
+            all-features)        cargo nextest run --all-features ;;
+          esac
+      - name: cargo test --doc
+        run: cargo test --doc
+
+  clippy:
+    name: Clippy (${{ matrix.features }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        features:
+          - default
+          - no-default-features
+          - all-features
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          components: clippy
+      - uses: Swatinem/rust-cache@v2
+      - name: cargo clippy
+        run: |
+          case "${{ matrix.features }}" in
+            default)             cargo clippy --all-targets -- -D warnings ;;
+            no-default-features) cargo clippy --all-targets --no-default-features -- -D warnings ;;
+            all-features)        cargo clippy --all-targets --all-features -- -D warnings ;;
+          esac
+
+  fmt:
+    name: Format
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          components: rustfmt
+      - run: cargo fmt -- --check
+
+  doc:
+    name: Doc
+    runs-on: ubuntu-latest
+    env:
+      RUSTDOCFLAGS: -D warnings
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+      - uses: Swatinem/rust-cache@v2
+      - run: cargo doc --no-deps --all-features
+
+  deny:
+    name: Cargo Deny
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: EmbarkStudios/cargo-deny-action@v2
+
+  miri:
+    name: Miri
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@nightly
+        with:
+          components: miri
+      - uses: Swatinem/rust-cache@v2
+      - uses: taiki-e/install-action@v2
+        with:
+          tool: nextest
+      - run: cargo +nightly miri nextest run --lib --test vectors
+      - run: cargo +nightly miri nextest run --lib --test vectors --no-default-features
+
+  fuzz:
+    name: Fuzz (60s per target)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@nightly
+      - uses: Swatinem/rust-cache@v2
+        with:
+          workspaces: |
+            .
+            fuzz
+      - uses: taiki-e/install-action@v2
+        with:
+          tool: cargo-fuzz
+      - name: Build fuzz targets
+        working-directory: fuzz
+        run: cargo +nightly fuzz build
+      - name: Smoke-run every target
+        working-directory: fuzz
+        run: |
+          for target in parse_command parse_radio_config cobs_decoder_feed cobs_roundtrip command_roundtrip cobs_decoder_multi_frame cobs_chunk_invariant; do
+            echo "── $target ──"
+            cargo +nightly fuzz run "$target" -- -max_total_time=60
+          done
+
+  mutants:
+    name: Mutants
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        features:
+          - default
+          - no-default-features
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+      - uses: Swatinem/rust-cache@v2
+      - uses: taiki-e/install-action@v2
+        with:
+          tool: cargo-mutants, nextest
+      - name: cargo mutants (${{ matrix.features }})
+        run: |
+          extra=""
+          [ "${{ matrix.features }}" = "no-default-features" ] && extra="--no-default-features"
+          cargo mutants --no-shuffle --test-tool=nextest $extra \
+            -E 'replace > with >= in Command::from_bytes' \
+            -E 'replace > with >= in CobsDecoder::feed' \
+            -E 'replace < with <= in cobs_encode_response'
+      - name: Upload mutants.out on failure
+        if: failure()
+        uses: actions/upload-artifact@v4
+        with:
+          name: mutants-${{ matrix.features }}
+          path: mutants.out