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/render/graphics.rs

@@ -5,7 +5,6 @@ use std::sync::Arc;
 use crate::entity::{EntityManager, EntityKind};
 use crate::world::cell::MaterialId;
 use crate::world::grid::Grid;
-use crate::world::material::MaterialRegistry;
 
 const CHAR_W: u32 = 16;
 const CHAR_H: u32 = 16;
@@ -306,7 +305,6 @@ impl GraphicsRenderer {
 
     pub fn render(&mut self, grid: &Grid, entities: &EntityManager, cam_x: i32, cam_y: i32) {
         self.check_resize();
-        let reg = MaterialRegistry::instance();
 
         let mut entity_map: std::collections::HashMap<(i32, i32), [u8; 4]> = std::collections::HashMap::new();
         for e in entities.all() {
@@ -474,7 +472,6 @@ impl GraphicsRenderer {
         for &v in &self.swapchain_image_views { unsafe { self.device.destroy_image_view(v, None); } }
 
         // Recreate swapchain
-        let qf_slice = [0u32]; // placeholder, not used with EXCLUSIVE
         let sci = vk::SwapchainCreateInfoKHR::default()
             .surface(self.surface)
             .min_image_count(caps.min_image_count.max(2))