Previously thumbnail was captured during sceneChange (before video starts
playing), so readyState was low and capture was skipped. Now capture on
videoEnd when frame is guaranteed visible, store in lastThumbnail, and
reuse for both auto-save (slot 0) and manual save.
- useGameEngine: auto-save to slot 0 on every sceneChange event
- useGameEngine: add resumeAutoSave() for continuing from auto-save
- useGameEngine: extract registerEvents() to share between start and resume
- SaveLoadMenu: show slot 0 as '自动存档' with distinct styling and read-only button
- App.vue: check auto-save on load, show '继续上次进度' button if available
- StateManager.dump(): console.table formatted dump of variables, flags, history
- window.__sm: exposes engine.stateManager in dev mode for console inspection
- 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
