refactor(lab): phase 1 — extract inline style block to /css/lab.css

lab.html 5180L → 4324L (-856L). All CSS moved to frontend/css/lab.css

(855L). Added <link> tag after ls.css for proper cascade.

Visual rendering unchanged — pure file move.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

plans/lab-split/phase-1-extract-style.md

@@ -1,6 +1,6 @@
 # Phase 1: Extract inline `<style>` → `frontend/css/lab.css`
 
-**Status:** ⬜ Not Started
+**Status:** ✅ Done
 **Parent plan:** [PLAN.md](./PLAN.md)
 **Domain:** frontend