test: 109 Rust tests + 14 JSON scenarios, polish physics

Tests added:
- physics_materials.rs: 15 tests (fire, smoke, steam, grass, dirt, bone, wood, stone, lava temp)
- physics_interactions.rs: 12 tests (lava+water, fire spread, acid interactions, sand+water)
- player_controls.rs: 12 tests (move L/R, jump, double jump, rapid input, wait, continuous)
- collision_robust.rs: 10 tests (walls, ceiling, sliding, corridor, slope, dirt wall, unstick)
- ragdoll_death.rs: 7 tests (death transition, ragdoll fall, progressive damage, corpse)
- determinism.rs: 8 tests (same seed, replay exact, replay partial, recording, 100-tick)
- edge_cases.rs: 19 tests (boundary, stress 20 goblins, fill/clear/paint, gravity, camera)
- 6 new JSON scenarios (fire chain, lava pool, sand bury, entity fall, steam, acid wall)

Fixes:
- AABB resolve_aabb_y: add vertical overlap check (was pushing player down when jumping)
- AABB resolve_aabb_x: velocity-aware resolution (player was stuck against walls)
- check_on_ground: use fractional position check (was always true)
- jump_force 1.5 (was 0.8) for 27-body entity
- Gravity read from self.verlet (was from clone, SetGravity didn't work)
- u8 overflow in lava color (saturating_add)

109 Rust tests, 14 JSON scenarios, 0 warnings, 0 failures

tests/integration.rs

@@ -1,6 +1,6 @@
 use verbatim::ai::GameSession;
 use verbatim::ai::AiAction;
-use verbatim::ai::{ReplayRecorder, ReplayPlayer};
+use verbatim::ai::ReplayPlayer;
 
 #[test]
 fn replay_deterministic() {