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/core/SceneManager.ts

@@ -1,4 +1,4 @@
-import type { GameData, SceneNode } from '../types'
+import type { GameData, SceneNode, Choice, Condition } from '../types'
 
 export class SceneManager {
   private scenes: Record<string, SceneNode> = {}
@@ -22,4 +22,30 @@ export class SceneManager {
   getAllSceneIds(): string[] {
     return Object.keys(this.scenes)
   }
+
+  getCandidateTargetIds(scene: SceneNode, evaluateCondition: (conds?: Condition[]) => boolean): string[] {
+    const targets: string[] = []
+
+    if (scene.choices) {
+      for (const choice of scene.choices) {
+        if (!choice.conditions || evaluateCondition(choice.conditions)) {
+          if (!targets.includes(choice.targetScene)) {
+            targets.push(choice.targetScene)
+          }
+        }
+      }
+    }
+
+    if (scene.nextScene && !targets.includes(scene.nextScene)) {
+      targets.push(scene.nextScene)
+    }
+
+    return targets
+  }
+
+  getCandidateUrls(scene: SceneNode, evaluateCondition: (conds?: Condition[]) => boolean): string[] {
+    return this.getCandidateTargetIds(scene, evaluateCondition)
+      .map(id => this.scenes[id]?.videoUrl)
+      .filter((url): url is string => !!url)
+  }
 }