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/input.rs

@@ -14,7 +14,6 @@ pub enum Action {
     MoveCameraRight,
     Paint(MaterialBrush),
     Quit,
-    None,
 }
 
 #[derive(Clone, Copy, PartialEq, Eq, Debug)]
@@ -221,16 +220,6 @@ impl InputHandler {
     pub fn jump_requested(&self) -> bool {
         self.jump_pressed
     }
-
-    pub fn release_all(&mut self) {
-        self.held.clear();
-        self.jump_pressed = false;
-    }
-
-    pub fn poll(&mut self) -> Action {
-        let actions = self.update();
-        actions.into_iter().next().unwrap_or(Action::None)
-    }
 }
 
 impl MaterialBrush {
@@ -251,19 +240,4 @@ impl MaterialBrush {
         }
     }
 
-    pub fn name(&self) -> &'static str {
-        match self {
-            MaterialBrush::Sand => "Sand",
-            MaterialBrush::Water => "Water",
-            MaterialBrush::Stone => "Stone",
-            MaterialBrush::Lava => "Lava",
-            MaterialBrush::Wood => "Wood",
-            MaterialBrush::Acid => "Acid",
-            MaterialBrush::Grass => "Grass",
-            MaterialBrush::Dirt => "Dirt",
-            MaterialBrush::Fire => "Fire",
-            MaterialBrush::Flesh => "Flesh",
-            MaterialBrush::Erase => "Erase",
-        }
-    }
 }