Docs: Restructure open plans to match spec (Preface → Spec → Design → Impl → Checklist)

- Rewrite project-analysis-improvements with Spec/Goal, Design, Implementation, Checklist sections; condense completed items (24/29) into preface summary, focus on 5 remaining items
- Rewrite mouse-support with full spec structure; consolidate Goal/Non-Goals/Constraints into Spec, merge Current State/Design Overview/API Sketch into Design, fold Phase Plan/Testing/Rollout into Implementation
- Rename 2026-02-26-mouse-support-plan.md to 2026-02-26-mouse-support.md (drop redundant -plan suffix per YYYY-MM-DD-kebab-case spec)

Author: phranck