Version Control: 场景版本与命名 Checkpoint

[DankerMu]

Part of #8

Dependencies: #10 (MVP-β 完成)

Description

实现场景自动版本管理、章节快照、命名 checkpoint 和回滚功能。

OpenSpec Spec: `version-control`

Tasks

• 2.1 Implement scene version management (auto-version on save)
• 2.2 Implement chapter snapshot on chapter_mark_done
• 2.3 Implement named checkpoint API (manual save points)
• 2.4 Implement rollback to any snapshot/checkpoint
• 2.5 Build version history panel (timeline view + diff)
• 2.6 Write tests for versioning, snapshot, rollback

Acceptance Criteria

• 每次保存自动创建新版本
• 章节完成时自动创建快照
• 用户可创建命名 checkpoint
• 支持回滚到任意版本/快照/checkpoint
• 版本历史面板支持 diff 查看