Add Vitest test suite with 184 tests covering all game engine systems

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/game-engine/vitest.config.ts

@@ -0,0 +1,7 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    include: ['src/**/*.test.ts'],
+  },
+});