refactor(lab): phase 2 — extract inline glue script to /js/labs/lab-glue.js

lab.html 4324L → 3499L (-825L). 824 lines of glue code moved.

Position preserved: lab-glue.js loads after lab-init.js, before

newton.js / forcesandbox.js / etc. — same execution order as inline.

node --check passes. /lab returns 200. /js/labs/lab-glue.js returns 200.

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

plans/lab-split/phase-2-extract-glue.md

@@ -1,6 +1,6 @@
 # Phase 2: Extract inline `<script>` glue → `frontend/js/labs/lab-glue.js`
 
-**Status:** ⬜ Not Started
+**Status:** ✅ Done
 **Parent plan:** [PLAN.md](./PLAN.md)
 **Domain:** frontend