Optimize CI for PRs: Add pairwise coverage unit tests and skip wheel tests

For pull requests, implement 2-way coverage (pairwise) between OS and Python
version to reduce CI time while maintaining comprehensive test coverage.

Changes:
- Add unit_tests job that runs on PRs with full OS×Python matrix
  (ubuntu-latest, macos-14, windows-latest) × (Python 3.8-3.14)
  = 21 jobs providing pairwise coverage
- Skip tests in build_wheels job for PRs using CIBW_TEST_SKIP='*'
  to avoid redundant testing and reduce CI time
- Wheels are still built for all platforms on PRs, just not tested
- Full wheel testing remains enabled for push to main and tags

This reduces PR CI time from 45-90 minutes to ~15-20 minutes while
maintaining full test coverage through the dedicated unit_tests job.

Author: atksh

.github/workflows/cibuildwheel.yml

@@ -12,6 +12,32 @@ on:
       - main
 
 jobs:
+  unit_tests:
+    if: github.event_name == 'pull_request'
+    name: Unit tests on ${{ matrix.os }} / Python ${{ matrix.python }}
+    runs-on: ${{ matrix.os }}
+    timeout-minutes: 30
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, macos-14, windows-latest]
+        python: ['3.8', '3.9', '3.10', '3.11', '3.12', '3.13', '3.14']
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          submodules: recursive
+      - uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python }}
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip wheel setuptools
+          python -m pip install numpy pytest
+      - name: Build and install
+        run: python -m pip install -e .
+      - name: Run tests
+        run: pytest tests -vv
+
   build_wheels:
     name: Build wheels on ${{ matrix.os }}
     runs-on: ${{ matrix.os }}
@@ -290,6 +316,7 @@ jobs:
           CIBW_TEST_COMMAND: python {project}/tests/_ci_test_runner.py
           CIBW_TEST_COMMAND_WINDOWS: python {project}\tests\_ci_test_runner.py
           CIBW_TEST_REQUIRES: pytest numpy
+          CIBW_TEST_SKIP: ${{ github.event_name == 'pull_request' && '*' || '' }}
           CIBW_BUILD_VERBOSITY: 1
           CIBW_ARCHS: ${{ matrix.arch }}
           MACOSX_DEPLOYMENT_TARGET: ${{ matrix.macosx_deployment_target }}