chore: upgrade vitest 3.2.4 → 4.0.18

Breaking changes addressed:
- vi.fn() mock factories: arrow → regular functions for constructor support
- Exclude dist/ from test resolution (vitest 4 simplified defaults)
- 672 tests pass, 0 tsc errors

vitest.config.ts

@@ -1,9 +1,13 @@
-import { defineConfig } from "vitest/config";
+import { configDefaults, defineConfig } from "vitest/config";
 
 export default defineConfig({
   test: {
     setupFiles: ["src/__tests__/setup.ts"],
     testTimeout: 15000,
     hookTimeout: 15000,
+    exclude: [
+      ...configDefaults.exclude,
+      "**/dist/**",
+    ],
   },
 });