fix: arcade mode crash — kaplay go() defers scene init to next frame

k.go('arcade') schedules the scene callback on frameEnd, not synchronously.
start() was called before fighters existed, causing "Cannot read properties
of undefined (reading 'obj')". Fix: await a readiness promise that resolves
once the scene callback has created the fighters.

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

frontend/src/game/arcade/types.ts

@@ -97,7 +97,7 @@ export interface ArcadeCallbacks {
 }
 
 export interface ArcadeSceneController {
-  start: () => void
+  start: () => Promise<void>
   pause: () => void
   resume: () => void
   destroy: () => void