feat: P1 core - seamless video switching, conditional branches, save/load

- VideoManager: A/B dual-buffered video with crossfade transitions and candidate preloading
- Engine: condition-based choice filtering, ChoiceSystem timer, resumeScene for save/load
- SceneManager: getCandidateUrls for preloading next scenes
- SaveSystem: Dexie.js IndexedDB multi-slot save/load
- ChoiceSystem: timed choices with countdown and auto-default on timeout
- GamePlayer: dual video elements with crossfade CSS
- ChoicePanel: timer progress bar and countdown text
- SaveLoadMenu: save/load UI component
- App.vue: menu trigger, dual video refs, save/load integration
- gameStore: timer state, saves list
- demo.json: conditional choice example (secret ending, requires trust >= 80)
- ROADMAP: mark P1 as completed

engine/core/Engine.ts

@@ -2,6 +2,7 @@ import type { SceneNode, Choice, EngineEvent } from '../types'
 import { SceneManager } from './SceneManager'
 import { VideoManager } from './VideoManager'
 import { StateManager } from './StateManager'
+import { ChoiceSystem } from '../systems/ChoiceSystem'
 
 type EventHandler = (...args: any[]) => void
 
@@ -9,15 +10,18 @@ export class Engine {
   sceneManager: SceneManager
   videoManager: VideoManager
   stateManager: StateManager
+  choiceSystem: ChoiceSystem
 
   private currentScene: SceneNode | null = null
   private events: Map<EngineEvent, Set<EventHandler>> = new Map()
-  private ended: boolean = false
+  private ended = false
+  private isInitialScene = true
 
   constructor() {
     this.sceneManager = new SceneManager()
     this.videoManager = new VideoManager()
     this.stateManager = new StateManager()
+    this.choiceSystem = new ChoiceSystem()
   }
 
   on(event: EngineEvent, handler: EventHandler) {
@@ -35,6 +39,7 @@ export class Engine {
 
   start() {
     this.ended = false
+    this.isInitialScene = true
     const startScene = this.sceneManager.getStartScene()
     this.goToScene(startScene)
   }
@@ -46,18 +51,42 @@ export class Engine {
       this.stateManager.apply(scene.onEnter)
     }
 
+    const preloadUrls = this.sceneManager.getCandidateUrls(
+      scene,
+      (conds) => conds ? this.stateManager.evaluate(conds) : true
+    )
+
     this.videoManager.onEnd(() => {
       this.emit('videoEnd', scene)
       this.onVideoEnd(scene)
     })
 
-    this.videoManager.play(scene.videoUrl)
+    if (this.isInitialScene) {
+      this.isInitialScene = false
+      this.videoManager.playInitial(scene.videoUrl, preloadUrls)
+    } else {
+      this.videoManager.switchTo(scene.videoUrl, preloadUrls)
+    }
+
     this.emit('sceneChange', scene)
   }
 
   private onVideoEnd(scene: SceneNode) {
-    if (scene.choices && scene.choices.length > 0) {
-      this.emit('choiceRequest', scene.choices)
+    const validChoices = this.getValidChoices(scene)
+
+    if (validChoices.length > 0) {
+      this.emit('choiceRequest', validChoices)
+
+      this.choiceSystem.start(
+        validChoices,
+        (timerState) => {
+          this.emit('choiceTimer', timerState)
+        },
+        (defaultChoice) => {
+          this.emit('choiceTimeout', defaultChoice)
+          this.makeChoice(defaultChoice)
+        }
+      )
     } else if (scene.nextScene) {
       const next = this.sceneManager.getScene(scene.nextScene)
       if (next) {
@@ -70,6 +99,13 @@ export class Engine {
     }
   }
 
+  getValidChoices(scene: SceneNode): Choice[] {
+    if (!scene.choices) return []
+    return scene.choices.filter((c) =>
+      !c.conditions || this.stateManager.evaluate(c.conditions)
+    )
+  }
+
   makeChoice(choice: Choice) {
     if (!this.currentScene) return
 
@@ -96,6 +132,36 @@ export class Engine {
     this.emit('gameEnd')
   }
 
+  resumeScene(sceneId: string, savedState: { variables: Record<string, number>; flags: string[]; history: any[] }) {
+    this.stateManager.variables = { ...savedState.variables }
+    this.stateManager.flags = new Set(savedState.flags)
+    this.stateManager.history = [...savedState.history]
+
+    const scene = this.sceneManager.getScene(sceneId)
+    if (!scene) {
+      this.endGame()
+      return
+    }
+
+    this.ended = false
+    this.isInitialScene = false
+
+    const preloadUrls = this.sceneManager.getCandidateUrls(
+      scene,
+      (conds) => conds ? this.stateManager.evaluate(conds) : true
+    )
+
+    this.videoManager.switchTo(scene.videoUrl, preloadUrls)
+
+    this.videoManager.onEnd(() => {
+      this.emit('videoEnd', scene)
+      this.onVideoEnd(scene)
+    })
+
+    this.currentScene = scene
+    this.emit('sceneChange', scene)
+  }
+
   destroy() {
     this.videoManager.detach()
     this.events.clear()