feat: 3D digital twin major rework - real models, clean labels, improved geometry
- Real STL Models: 6 models loaded (55%), fallback for 5 (45%)
- Clean Labels: 12+ → 5 default, step-based, technical toggle
- Geometry: Gate lift, pusher plates, conveyor rollers, roll-cage wireframes
- Demo Director: 12 steps, camera presets, auto-play logic (not integrated)
- Tests: +16 new (43/43 green), Build ✅, Docker ✅, Domains ✅
Remaining: Auto demo UI integration, camera transitions, docs updates, Visual QA
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