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

src/entity/player.rs

@@ -4,7 +4,6 @@ pub struct Player {
     pub entity_id: EntityId,
     pub move_speed: f32,
     pub jump_force: f32,
-    pub move_dir: i32,
 }
 
 impl Player {
@@ -14,7 +13,6 @@ impl Player {
             entity_id: id,
             move_speed: 0.5,
             jump_force: 1.5,
-            move_dir: 0,
         }
     }
 
@@ -25,21 +23,18 @@ impl Player {
     }
 
     pub fn move_left(&mut self, manager: &mut EntityManager) {
-        self.move_dir = -1;
         if let Some(e) = manager.get_mut(self.entity_id) {
             e.set_horizontal_vel(-self.move_speed);
         }
     }
 
     pub fn move_right(&mut self, manager: &mut EntityManager) {
-        self.move_dir = 1;
         if let Some(e) = manager.get_mut(self.entity_id) {
             e.set_horizontal_vel(self.move_speed);
         }
     }
 
     pub fn stop_horizontal(&mut self, manager: &mut EntityManager) {
-        self.move_dir = 0;
         if let Some(e) = manager.get_mut(self.entity_id) {
             e.set_horizontal_vel(0.0);
         }
@@ -57,10 +52,6 @@ impl Player {
         manager.get(self.entity_id)
     }
 
-    pub fn entity_mut<'a>(&self, manager: &'a mut EntityManager) -> Option<&'a mut Entity> {
-        manager.get_mut(self.entity_id)
-    }
-
     pub fn center(&self, manager: &EntityManager) -> (f32, f32) {
         manager.get(self.entity_id).map(|e| e.center()).unwrap_or((0.0, 0.0))
     }