feat: configure Vitest testing framework

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

vitest.config.ts

@@ -0,0 +1,14 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    globals: true,
+    environment: 'node',
+    include: ['tests/**/*.{test,spec}.{ts,tsx}'],
+    exclude: ['node_modules', 'dist', 'out'],
+    coverage: {
+      provider: 'v8',
+      reporter: ['text', 'json', 'html'],
+    },
+  },
+});