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

public/scenes/demo.json

@@ -68,6 +68,23 @@
     "trust_ending": {
       "id": "trust_ending",
       "videoUrl": "/videos/trust_ending.mp4",
+      "choices": [
+        {
+          "text": "开启信任的旅程（需要 trust >= 80）",
+          "targetScene": "secret_ending",
+          "conditions": [
+            { "variable": "trust", "op": ">=", "value": 80 }
+          ]
+        },
+        {
+          "text": "离开这里",
+          "targetScene": "alone_ending"
+        }
+      ]
+    },
+    "secret_ending": {
+      "id": "secret_ending",
+      "videoUrl": "/videos/continue_ending.mp4",
       "choices": []
     },
     "alone_ending": {