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

engine/systems/SaveSystem.ts

@@ -0,0 +1,72 @@
+import Dexie, { type Table } from 'dexie'
+import type { SaveData } from '../types'
+
+interface SaveRecord {
+  id?: number
+  slot: number
+  timestamp: number
+  currentScene: string
+  variables: string
+  flags: string
+  history: string
+}
+
+class SaveDB extends Dexie {
+  saves!: Table<SaveRecord, number>
+
+  constructor() {
+    super('MovieGameSaves')
+    this.version(1).stores({
+      saves: '++id, slot',
+    })
+  }
+}
+
+const db = new SaveDB()
+
+export class SaveSystem {
+  async save(slot: number, data: Omit<SaveData, 'slot' | 'thumbnail'>): Promise<void> {
+    const record: SaveRecord = {
+      slot,
+      timestamp: Date.now(),
+      currentScene: data.currentScene,
+      variables: JSON.stringify(data.variables),
+      flags: JSON.stringify(data.flags),
+      history: JSON.stringify(data.history),
+    }
+
+    const existing = await db.saves.where('slot').equals(slot).first()
+    if (existing) {
+      await db.saves.update(existing.id!, record)
+    } else {
+      await db.saves.add(record)
+    }
+  }
+
+  async load(slot: number): Promise<SaveData | null> {
+    const record = await db.saves.where('slot').equals(slot).first()
+    if (!record) return null
+
+    return {
+      slot: record.slot,
+      timestamp: record.timestamp,
+      currentScene: record.currentScene,
+      variables: JSON.parse(record.variables),
+      flags: JSON.parse(record.flags),
+      history: JSON.parse(record.history),
+    }
+  }
+
+  async listSlots(): Promise<{ slot: number; timestamp: number; sceneLabel: string }[]> {
+    const records = await db.saves.orderBy('slot').toArray()
+    return records.map((r) => ({
+      slot: r.slot,
+      timestamp: r.timestamp,
+      sceneLabel: r.currentScene,
+    }))
+  }
+
+  async delete(slot: number): Promise<void> {
+    await db.saves.where('slot').equals(slot).delete()
+  }
+}