fix: Repair test harness coverage, excludes, and timer overhead

- Add @vitest/coverage-v8 dep so `npm run test:coverage` actually works
- Add exclude patterns to vitest config (node_modules, dist, packages)
- Replace dynamic import('vitest') in advanceTimers with direct vi import

vitest.config.ts

@@ -9,6 +9,7 @@ export default defineConfig({
     },
     // Test file pattern
     include: ['**/*.test.ts'],
+    exclude: ['**/node_modules/**', '**/dist/**', 'packages/**'],
     // TypeScript support uses tsconfig.json automatically
     // Coverage reporter (optional, for future use)
     coverage: {