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)

vitest.config.ts

@@ -1,7 +1,19 @@
 import { defineConfig } from 'vitest/config';
+import path from 'node:path';
 
 export default defineConfig({
+  resolve: {
+    alias: {
+      '@': path.resolve(__dirname, 'src'),
+    },
+  },
   test: {
     globals: true,
+    include: ['test/**/*.test.ts'],
+    coverage: {
+      provider: 'v8',
+              include: ['src/**/*.ts'],
+      exclude: ['src/**/index.ts'],
+    },
   },
 });