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/ChoicePanel.vue

@@ -3,15 +3,35 @@ import type { Choice } from '@engine/types'
 
 const props = defineProps<{
   choices: Choice[]
+  timerTotal: number
+  timerRemaining: number
 }>()
 
 const emit = defineEmits<{
   choose: [index: number]
 }>()
+
+function timerPercent(): number {
+  if (props.timerTotal <= 0) return 0
+  return (props.timerRemaining / props.timerTotal) * 100
+}
+
+function timerClass(): string {
+  if (props.timerRemaining <= 3) return 'danger'
+  return ''
+}
 </script>
 
 <template>
   <div class="choice-panel" v-if="choices.length > 0">
+    <div class="timer-bar" v-if="timerTotal > 0">
+      <div
+        class="timer-fill"
+        :class="timerClass()"
+        :style="{ width: timerPercent() + '%' }"
+      ></div>
+      <span class="timer-text">{{ timerRemaining.toFixed(1) }}s</span>
+    </div>
     <div class="choice-prompt">做出你的选择</div>
     <div class="choice-list">
       <button
@@ -33,10 +53,38 @@ const emit = defineEmits<{
   left: 0;
   right: 0;
   background: linear-gradient(transparent, rgba(0, 0, 0, 0.85));
-  padding: 40px 20px 30px;
+  padding: 20px 20px 30px;
   z-index: 10;
 }
 
+.timer-bar {
+  height: 4px;
+  background: rgba(255, 255, 255, 0.15);
+  border-radius: 2px;
+  margin-bottom: 16px;
+  position: relative;
+  overflow: hidden;
+}
+
+.timer-fill {
+  height: 100%;
+  background: rgba(255, 255, 255, 0.6);
+  transition: width 0.1s linear;
+  border-radius: 2px;
+}
+
+.timer-fill.danger {
+  background: #e74c3c;
+}
+
+.timer-text {
+  position: absolute;
+  top: -18px;
+  right: 0;
+  font-size: 12px;
+  color: #aaa;
+}
+
 .choice-prompt {
   color: #ccc;
   font-size: 14px;

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>

src/components/SaveLoadMenu.vue

@@ -0,0 +1,157 @@
+<script setup lang="ts">
+import { ref, onMounted } from 'vue'
+import type { SlotInfo } from '@/stores/gameStore'
+
+const props = defineProps<{
+  saves: SlotInfo[]
+}>()
+
+const emit = defineEmits<{
+  save: [slot: number]
+  load: [slot: number]
+  close: []
+}>()
+
+const maxSlots = 5
+</script>
+
+<template>
+  <div class="save-overlay" @click.self="emit('close')">
+    <div class="save-panel">
+      <h2 class="save-title">存档 / 读档</h2>
+
+      <div class="slot-list">
+        <div
+          v-for="slot in maxSlots"
+          :key="slot"
+          class="save-slot"
+        >
+          <div class="slot-label">存档 {{ slot }}</div>
+          <div class="slot-info" v-if="saves.find(s => s.slot === slot)">
+            {{ saves.find(s => s.slot === slot)!.sceneLabel }}
+          </div>
+          <div class="slot-info empty" v-else>空</div>
+          <div class="slot-actions">
+            <button class="slot-btn save-btn" @click="emit('save', slot)">保存</button>
+            <button
+              class="slot-btn load-btn"
+              :disabled="!saves.find(s => s.slot === slot)"
+              @click="emit('load', slot)"
+            >
+              读取
+            </button>
+          </div>
+        </div>
+      </div>
+
+      <button class="close-btn" @click="emit('close')">关闭</button>
+    </div>
+  </div>
+</template>
+
+<style scoped>
+.save-overlay {
+  position: fixed;
+  inset: 0;
+  background: rgba(0, 0, 0, 0.85);
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  z-index: 200;
+}
+
+.save-panel {
+  background: #1a1a2e;
+  border: 1px solid rgba(255, 255, 255, 0.15);
+  border-radius: 8px;
+  padding: 32px;
+  min-width: 400px;
+  max-width: 500px;
+}
+
+.save-title {
+  text-align: center;
+  font-size: 20px;
+  font-weight: 400;
+  letter-spacing: 2px;
+  margin-bottom: 24px;
+  color: #ddd;
+}
+
+.slot-list {
+  display: flex;
+  flex-direction: column;
+  gap: 10px;
+}
+
+.save-slot {
+  display: flex;
+  align-items: center;
+  gap: 12px;
+  padding: 12px 16px;
+  background: rgba(255, 255, 255, 0.05);
+  border: 1px solid rgba(255, 255, 255, 0.1);
+  border-radius: 4px;
+}
+
+.slot-label {
+  font-size: 14px;
+  color: #aaa;
+  white-space: nowrap;
+}
+
+.slot-info {
+  flex: 1;
+  font-size: 13px;
+  color: #ccc;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+.slot-info.empty {
+  color: #555;
+}
+
+.slot-actions {
+  display: flex;
+  gap: 6px;
+}
+
+.slot-btn {
+  padding: 6px 14px;
+  font-size: 13px;
+  border: 1px solid rgba(255, 255, 255, 0.2);
+  border-radius: 3px;
+  cursor: pointer;
+  background: rgba(255, 255, 255, 0.08);
+  color: #ddd;
+  transition: background 0.2s;
+}
+
+.slot-btn:hover:not(:disabled) {
+  background: rgba(255, 255, 255, 0.18);
+}
+
+.slot-btn:disabled {
+  opacity: 0.35;
+  cursor: default;
+}
+
+.close-btn {
+  display: block;
+  margin: 24px auto 0;
+  padding: 10px 32px;
+  font-size: 14px;
+  color: #888;
+  background: rgba(255, 255, 255, 0.06);
+  border: 1px solid rgba(255, 255, 255, 0.15);
+  border-radius: 4px;
+  cursor: pointer;
+}
+
+.close-btn:hover {
+  color: #ccc;
+  background: rgba(255, 255, 255, 0.1);
+}
+</style>