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

vitest.config.ts

@@ -0,0 +1,16 @@
+import { defineConfig } from 'vitest/config'
+
+export default defineConfig({
+  test: {
+    globals: true,
+    environment: 'node',
+    include: ['src/**/__tests__/**/*.test.{ts,tsx}'],
+    coverage: {
+      provider: 'v8',
+      reporter: ['text', 'text-summary', 'json-summary'],
+      include: ['src/lib/**/*.ts'],
+      exclude: ['src/lib/__tests__/**'],
+      // Thresholds intentionally unset — coverage is tracked, not enforced yet.
+    },
+  },
+})