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

package.json

@@ -50,6 +50,7 @@
     "@types/better-sqlite3": "^7.6.13",
     "@types/js-yaml": "^4.0.9",
     "@types/node": "^22.10.7",
+    "@vitest/coverage-v8": "^4.0.18",
     "drizzle-kit": "^0.31.8",
     "pino-pretty": "^13.1.3",
     "rimraf": "^6.0.1",