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

src/components/GamePlayer.vue

@@ -2,39 +2,40 @@
 import { ref, onMounted } from 'vue'
 
 const emit = defineEmits<{
-  videoReady: [el: HTMLVideoElement]
+  videoReady: [elA: HTMLVideoElement, elB: HTMLVideoElement]
 }>()
 
-const videoRef = ref<HTMLVideoElement | null>(null)
+const videoARef = ref<HTMLVideoElement | null>(null)
+const videoBRef = ref<HTMLVideoElement | null>(null)
 
 onMounted(() => {
-  if (videoRef.value) {
-    emit('videoReady', videoRef.value)
+  if (videoARef.value && videoBRef.value) {
+    emit('videoReady', videoARef.value, videoBRef.value)
   }
 })
 
-defineExpose({ videoRef })
+defineExpose({ videoARef, videoBRef })
 </script>
 
 <template>
   <div class="game-player">
-    <video ref="videoRef" class="player-video" preload="auto"></video>
+    <video ref="videoARef" class="player-video" preload="auto"></video>
+    <video ref="videoBRef" class="player-video" preload="auto"></video>
   </div>
 </template>
 
 <style scoped>
 .game-player {
+  position: relative;
   width: 100%;
   height: 100%;
   background: #000;
-  display: flex;
-  align-items: center;
-  justify-content: center;
+  overflow: hidden;
 }
 
 .player-video {
   max-width: 100%;
   max-height: 100%;
-  object-fit: contain;
+  will-change: opacity;
 }
 </style>