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/components/ArcadeViewer.vue

@@ -151,12 +151,12 @@ async function initScene(): Promise<void> {
     comboTimer = setTimeout(() => { comboInfo.value = null }, 1200)
   })
 
-  // Start
+  // Start (awaits scene readiness — kaplay defers scene init to next frame)
   showAnnouncement('ROUND 1', 1200)
   setTimeout(() => {
     showAnnouncement('FIGHT!', 800)
   }, 1300)
-  scene.start()
+  await scene.start()
 
   // Start input polling
   inputPollId = requestAnimationFrame(pollInput)

frontend/src/game/ArcadeScene.ts

@@ -156,6 +156,11 @@ export async function createArcadeScene(config: ArcadeConfig): Promise<ArcadeSce
   let fighter1: FighterInstance
   let fighter2: FighterInstance
 
+  // Scene readiness — k.go() defers the scene callback to the next frame end,
+  // so we need to wait for fighters to be created before start() can run.
+  let sceneReady: () => void
+  const sceneReadyPromise = new Promise<void>(resolve => { sceneReady = resolve })
+
   // ═══════════════════════════════════════════════════════════════
   // Scene Setup
   // ═══════════════════════════════════════════════════════════════
@@ -180,6 +185,9 @@ export async function createArcadeScene(config: ArcadeConfig): Promise<ArcadeSce
     fighter1 = createFighter('p1', P1_START_X, 1, player1.name)
     fighter2 = createFighter('p2', P2_START_X, 2, player2.name)
 
+    // Signal that the scene is ready (fighters exist)
+    sceneReady()
+
     // ═══════════════════════════════════════════════════════════
     // Main Game Loop
     // ═══════════════════════════════════════════════════════════
@@ -489,7 +497,8 @@ export async function createArcadeScene(config: ArcadeConfig): Promise<ArcadeSce
   k.go('arcade')
 
   return {
-    start() {
+    async start() {
+      await sceneReadyPromise
       matchOver = false
       p1Wins = 0
       p2Wins = 0

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