chore: complete v1.5 milestone - B站字幕简化处理

- Archive roadmap to milestones/v1.5-ROADMAP.md
- Archive requirements to milestones/v1.5-REQUIREMENTS.md
- Update ROADMAP.md with collapsed v1.5 entry
- Update PROJECT.md with validated requirements
- Add semantic segmentation to backlog

Delivered: B站视频简化处理流程

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.planning/STATE.md

@@ -4,7 +4,7 @@ milestone: v1.4
 milestone_name: Zhihu Anti-Crawler
 status: completed
 stopped_at: Completed 12-02 Pipeline Integration
-last_updated: "2026-03-19T16:06:58.268Z"
+last_updated: "2026-03-21T15:24:05.123Z"
 last_activity: 2026-03-19 — Completed 12-02 Pipeline Integration
 progress:
   total_phases: 12