Demo 3: Tumble Tower — Stage 0 physics (2D AABB stacking) #231

Goal: implement a deterministic 2D rigid-body-ish simulation with AABB blocks and gravity.

Scope

• Fixed tick dt.
• Bodies: axis-aligned rectangles only.
• Gravity, velocity integration.
• Collisions with static floor/walls and between blocks.
• Deterministic contact resolution ordering (stable sort key).

Acceptance criteria

• Same seed + same inputs => identical per-tick fingerprints across runs.
• A small suite of tests pin stability (stacking does not drift across runs; collision ordering stable).