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-lock.json

@@ -8,6 +8,7 @@
       "name": "moviegame",
       "version": "0.1.0",
       "dependencies": {
+        "dexie": "^4.4.3",
         "pinia": "^2.1.0",
         "vue": "^3.4.0"
       },
@@ -965,6 +966,11 @@
       "integrity": "sha512-e/1zu3xH5MQryN2zdVaF0OrdNLUbvWxzMbi+iNA6Bky7l1RoP8a2fIbRocyHclXt/arDrrR6lL3TqFD9pMQTsg==",
       "dev": true
     },
+    "node_modules/dexie": {
+      "version": "4.4.3",
+      "resolved": "https://registry.npmjs.org/dexie/-/dexie-4.4.3.tgz",
+      "integrity": "sha512-N+3IGQ3HPlyO2YAkntGAwitm42BpBGV86MttzUMiRzWLa4NGh0pltVRcUVF4ybL/OnXjCrr9k7SDPIKkFYP2Lg=="
+    },
     "node_modules/entities": {
       "version": "7.0.1",
       "resolved": "https://registry.npmjs.org/entities/-/entities-7.0.1.tgz",