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/stores/gameStore.ts

@@ -2,10 +2,19 @@ import { defineStore } from 'pinia'
 import { ref, shallowRef } from 'vue'
 import type { SceneNode, Choice } from '@engine/types'
 
+export interface SlotInfo {
+  slot: number
+  timestamp: number
+  sceneLabel: string
+}
+
 export const useGameStore = defineStore('game', () => {
   const currentScene = shallowRef<SceneNode | null>(null)
   const choices = ref<Choice[]>([])
   const gameEnded = ref(false)
+  const timerTotal = ref(0)
+  const timerRemaining = ref(0)
+  const saves = ref<SlotInfo[]>([])
 
   function setScene(scene: SceneNode) {
     currentScene.value = scene
@@ -23,5 +32,23 @@ export const useGameStore = defineStore('game', () => {
     gameEnded.value = val
   }
 
-  return { currentScene, choices, gameEnded, setScene, setChoices, clearChoices, setGameEnded }
+  function setTimer(total: number, remaining: number) {
+    timerTotal.value = total
+    timerRemaining.value = remaining
+  }
+
+  function clearTimer() {
+    timerTotal.value = 0
+    timerRemaining.value = 0
+  }
+
+  function setSaves(list: SlotInfo[]) {
+    saves.value = list
+  }
+
+  return {
+    currentScene, choices, gameEnded, timerTotal, timerRemaining, saves,
+    setScene, setChoices, clearChoices, setGameEnded,
+    setTimer, clearTimer, setSaves,
+  }
 })