feat(setup/test-infrastructure): configure Vitest with shared test fixtures and helpers

- Add test:coverage script and @vitest/coverage-v8 dev dependency
- Update vitest.config.ts with @/ path alias and v8 coverage config
- Create test/fixtures/graphs.ts with 5 fixture graphs (linearChain,
  diamond, mixedCategory, cyclic, largeGraph) and createTaskGraph helper
- Expand graph.test.ts with 26 fixture validation tests
- 30 tests passing, CI-compatible output (vitest run)

package.json

@@ -25,6 +25,7 @@
     "build": "tsc",
     "test": "vitest run",
     "test:watch": "vitest",
+    "test:coverage": "vitest run --coverage",
     "lint": "tsc --noEmit",
     "prepublishOnly": "npm run build"
   },
@@ -37,20 +38,21 @@
   ],
   "license": "MIT",
   "dependencies": {
+    "@alkdev/typebox": "^0.34.49",
     "graphology": "^0.26.0",
+    "graphology-components": "^1.5.4",
     "graphology-dag": "^0.4.1",
     "graphology-metrics": "^2.4.0",
-    "graphology-components": "^1.5.4",
     "graphology-operators": "^1.6.1",
-    "@alkdev/typebox": "^0.34.49",
     "yaml": "^2.8.3"
   },
   "devDependencies": {
+    "@types/node": "^22.0.0",
+    "@vitest/coverage-v8": "^3.2.4",
     "typescript": "^5.7.0",
-    "vitest": "^3.1.0",
-    "@types/node": "^22.0.0"
+    "vitest": "^3.1.0"
   },
   "engines": {
     "node": ">=18.0.0"
   }
-}
+}