doco: C4收口 + A稿解析器升级(LLM骨架) + ep002/003/004骨架

- 新增 fusion_align.py(C4 compose + skeleton 提取/校验/预览)与 doco skeleton 命令
- ep001 C4 产物入库: 融合A稿.docx / 融合B稿.txt / c4_alignment.csv / fusion_review.csv
- ep002/003/004: A稿 + 分段骨架(json+预览), 上游待跑
- .gitignore 增 .c3_cache/.c4_cache (可重生中间缓存不入库)
- 更新 doco/CLAUDE.md 交接备注

.gitignore

@@ -75,3 +75,7 @@ doco/.env
 
 # Python build artifacts
 *.egg-info/
+
+# Doco AI 断点缓存(可重生,不入库)
+doco/programs/*/.c3_cache/
+doco/programs/*/.c4_cache/