Branch Writing: 分支试写与合并

[DankerMu]

Part of #9

Dependencies: #17 (Version Control)

Description

实现从任意场景/章节创建分支、并行编辑 A/B 故事线、选择胜出分支合并回主线。

OpenSpec Spec: `branch-writing`

Tasks

• 3.1 Implement branch creation from any scene/chapter
• 3.2 Implement parallel branch editing (A/B storylines)
• 3.3 Implement branch merge (select winner as mainline)
• 3.4 Build branch comparison UI (side-by-side diff)
• 3.5 Write tests for branch CRUD and merge logic

Acceptance Criteria

• 可从任意场景创建分支
• 多个分支可并行编辑
• 合并时选择胜出分支，创建新主线版本
• Side-by-side diff 对比分支差异