[squad:plan] Lattice Hollow — M1 Prototype: "Cold Lamp" (Fenster)

**Owner:** Fenster — Engineering

Part of the Lattice Hollow plan (#58481).

Goals:
- Prove the core loop of movement, room navigation, and light-routing puzzle interaction.
- Deliverables: controllable player, one test chamber, mirror/prism/node interactions, win-state reset flow.
- Technical notes on engine choice, scene structure, and input architecture.
- Keep scope contained — explicit non-goals to preserve prototype speed.

Depends on parent concept approval.

Start with the parent concept approval in issue #58481 and determine the engine, scene structure, and input architecture before implementation. Done means a contained prototype with controllable movement, one test chamber, mirror/prism/node interactions, and a win-state reset flow, while respecting the stated non-goals.

Written by the indexing model from the issue text.