refactor: code cleanup — dead code, warnings, duplication

Dead code removed (21 methods, 4 fields, 3 constants):
- Cell: MaterialId::ALL, MaterialId::from_u8 (unsafe transmute)
- Grid: get_mut, clear, fill_rect, swap, dump_region, next buffer field
- Entity: move_center, EntityManager::iter_mut
- Player: move_dir field, entity_mut
- VerletSolver: step, SubBody::add_vel, SubBody::apply_force
- CellularAutomaton: tick_count
- InputHandler: release_all, poll, Action::None
- WindowInput: clear
- GameSession: perform_action_and_step, is_recording, grid_mut
- ReplayPlayer: from_recording
- Material: empty()
- VulkanRenderer: tick_count field
- MaterialBrush: name()

Warnings fixed:
- Remove unused MaterialRegistry imports from renderers
- Remove unused reg variables in terminal/vulkan/graphics
- Remove unused water_surface in game.rs
- Remove unused p/y_death in tests
- Remove unused qf_slice in graphics.rs

Duplication eliminated:
- main.rs: run_ascii_mode + run_graphics_mode → generic run_gpu_mode<R: GpuRenderer>
  ~140 lines of duplicated event loop code removed
- GpuRenderer trait unifies VulkanRenderer and GraphicsRenderer API

Unsafe code fixed:
- rand_u8: static mut + unsafe → AtomicU8 + fetch_add (thread-safe)

Module cleanup:
- world/mod.rs: removed all unused re-exports
- physics/mod.rs: removed all unused re-exports
- entity/mod.rs: removed unused Entity/EntityId re-exports

Result: ~6500 → ~5964 lines, 0 non-deprecation warnings, 109 tests pass

tests/edge_cases.rs

@@ -166,7 +166,6 @@ fn center_camera_on_player() {
     s.perform_action(&AiAction::SetCamera { x: 0, y: 0 });
     s.step(30);
     s.perform_action(&AiAction::CenterCamera);
-    let p = s.get_player().unwrap();
     assert!(s.game.cam_x > 0, "camera should move from 0 toward player: got {}", s.game.cam_x);
     assert!(s.game.cam_y > 0, "camera should move from 0 toward player: got {}", s.game.cam_y);
 }

tests/ragdoll_death.rs

@@ -36,11 +36,6 @@ fn ragdoll_falls_after_death() {
     let y_before = g.pos[1];
 
     s.perform_action(&AiAction::DamageEntity { id: g.id, amount: 100.0 });
-    s.step(1);
-    let y_death = {
-        let e = s.get_entities().into_iter().find(|e| e.id == g.id).unwrap();
-        e.pos[1]
-    };
     s.step(30);
     let y_after = {
         let e = s.get_entities().into_iter().find(|e| e.id == g.id).unwrap();