ci: Add code coverage job with cargo-llvm-cov

[Coldaine]

Source

PR #234 - can be closed after this issue exists.

What it does

Adds a CI job that generates code coverage using cargo-llvm-cov and uploads to Codecov.

Key code (CI workflow snippet)

code-coverage:
  name: Code Coverage
  runs-on: [self-hosted, Linux, X64, fedora, nobara]
  needs: [setup-whisper-dependencies]
  steps:
    - uses: actions/checkout@v5
    - name: Set up Rust toolchain
      uses: actions-rust-lang/setup-rust-toolchain@v1
      with:
        toolchain: stable
        components: llvm-tools-preview
    - name: Setup ColdVox
      uses: ./.github/actions/setup-coldvox
      with:
        skip-toolchain: "true"
    - name: Install cargo-llvm-cov
      uses: taiki-e/install-action@cargo-llvm-cov
    - name: Generate code coverage
      env:
        WHISPER_MODEL_PATH: ${{ needs.setup-whisper-dependencies.outputs.model_path }}
        WHISPER_MODEL_SIZE: ${{ needs.setup-whisper-dependencies.outputs.model_size }}
      run: |
        pip install faster-whisper
        export PYTHONPATH=$(python3 -c "import site; print(site.getsitepackages()[0])")
        cargo llvm-cov --workspace --lcov --output-path lcov.info
    - name: Upload coverage to Codecov
      uses: codecov/codecov-action@v4
      with:
        token: ${{ secrets.CODECOV_TOKEN }}
        files: lcov.info
        fail_ci_if_error: true

Implementation notes

• Requires CODECOV_TOKEN secret (or use tokenless upload for public repos)
• Adds CI runtime overhead
• Should be added after main test suite is stable

Status

Defer until CI stabilizes; snippet ready to paste into ci.yml