Add CI workflow for running tests on PRs

- Run tests on Ubuntu, Windows, and macOS
- Test against Python 3.9, 3.10, 3.11, 3.12
- Build .NET binaries before running tests
- Include coverage reporting via Codecov
- Add separate unit-test-only job for quick feedback
- Add linting check with ruff
- Add build verification job

Author: claude

.github/workflows/ci.yml

@@ -0,0 +1,132 @@
+name: CI
+
+on:
+  push:
+    branches: [main, master]
+  pull_request:
+    branches: [main, master]
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+        python-version: ['3.9', '3.10', '3.11', '3.12']
+
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Set up Python ${{ matrix.python-version }}
+      uses: actions/setup-python@v5
+      with:
+        python-version: ${{ matrix.python-version }}
+
+    - name: Set up .NET
+      uses: actions/setup-dotnet@v4
+      with:
+        dotnet-version: '8.0.x'
+
+    - name: Install Python dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install pytest pytest-cov hatch
+
+    - name: Build engine binaries
+      run: python build_differ.py
+
+    - name: Install package
+      run: pip install -e .
+
+    - name: Run tests with coverage
+      run: |
+        pytest tests/ -v --cov=python_redlines --cov-report=xml --cov-report=term-missing
+
+    - name: Upload coverage reports
+      uses: codecov/codecov-action@v4
+      if: matrix.os == 'ubuntu-latest' && matrix.python-version == '3.11'
+      with:
+        file: ./coverage.xml
+        fail_ci_if_error: false
+
+  # Quick test without binaries (unit tests only)
+  test-unit-only:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Set up Python
+      uses: actions/setup-python@v5
+      with:
+        python-version: '3.11'
+
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install pytest pytest-cov
+
+    - name: Install package (without building binaries)
+      env:
+        SKIP_BINARY_BUILD: "1"
+      run: pip install -e .
+
+    - name: Run unit tests (no integration tests)
+      run: |
+        pytest tests/ -v --ignore=tests/test_engines.py -k "not integration"
+
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Set up Python
+      uses: actions/setup-python@v5
+      with:
+        python-version: '3.11'
+
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install ruff
+
+    - name: Check code formatting
+      run: ruff check src/ tests/ --output-format=github
+      continue-on-error: true
+
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Set up Python
+      uses: actions/setup-python@v5
+      with:
+        python-version: '3.11'
+
+    - name: Set up .NET
+      uses: actions/setup-dotnet@v4
+      with:
+        dotnet-version: '8.0.x'
+
+    - name: Install build dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install hatch hatchling
+
+    - name: Build package
+      run: hatch build
+
+    - name: Check package
+      run: |
+        pip install twine
+        twine check dist/*
+
+    - name: Upload build artifacts
+      uses: actions/upload-artifact@v4
+      with:
+        name: dist
+        path: dist/