ci: add test coverage reporting with v8 provider

- Add vitest.config.ts with v8 coverage provider
- Add npm run test:coverage script
- Add coverage step to CI (Node 22, reports in job summary)
- Coverage tracks src/lib/ utilities (currently ~21% lines)
- Install @vitest/coverage-v8 dev dependency

.github/workflows/ci.yml

@@ -31,6 +31,15 @@ jobs:
       - name: Test
         run: npm test
 
+      - name: Test coverage
+        if: matrix.node-version == 22
+        run: |
+          npm run test:coverage
+          echo "## Test Coverage" >> $GITHUB_STEP_SUMMARY
+          echo '```' >> $GITHUB_STEP_SUMMARY
+          npm run test:coverage 2>&1 | grep -A20 'Coverage report' >> $GITHUB_STEP_SUMMARY
+          echo '```' >> $GITHUB_STEP_SUMMARY
+
       - name: Type check
         run: npx tsc --noEmit