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

package.json

@@ -9,8 +9,9 @@
     "preview": "vite preview"
   },
   "dependencies": {
-    "vue": "^3.4.0",
-    "pinia": "^2.1.0"
+    "dexie": "^4.4.3",
+    "pinia": "^2.1.0",
+    "vue": "^3.4.0"
   },
   "devDependencies": {
     "@vitejs/plugin-vue": "^5.0.0",