Demo 3: Tumble Tower — visualization (2D view + debug overlays)

Goal: make the physics demo watchable and debuggable.

Options
- Terminal renderer (fast, deterministic, low deps) for early stages.
- Later: lightweight 2D viewer (wasm/web or native) with overlays.

Overlays to support debugging
- contact points
- velocities
- sleep state
- per-tick fingerprint

Acceptance criteria
- Viewer is explicitly non-authoritative: derived from state only.
