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))

src/render/terminal.rs

@@ -11,7 +11,6 @@ use crate::entity::EntityManager;
 use crate::render::Renderer;
 use crate::world::cell::MaterialId;
 use crate::world::grid::Grid;
-use crate::world::material::MaterialRegistry;
 
 pub struct TerminalRenderer {
     width: usize,
@@ -70,7 +69,6 @@ impl Renderer for TerminalRenderer {
             return Ok(());
         }
 
-        let reg = MaterialRegistry::instance();
         let mut out = stdout();
         let mut frame: Vec<(char, (u8, u8, u8), (u8, u8, u8))>;
         let total = self.width * self.height;
@@ -80,13 +78,12 @@ impl Renderer for TerminalRenderer {
             for dx in 0..self.width {
                 let wx = cam_x + dx as i32;
                 let wy = cam_y + dy as i32;
+                let idx = dy * self.width + dx;
                 if !grid.in_bounds(wx, wy) {
-                    let idx = dy * self.width + dx;
                     frame[idx] = ('?', (80, 80, 80), (10, 10, 15));
                     continue;
                 }
                 let cell = grid.get(wx, wy);
-                let idx = dy * self.width + dx;
                 if cell.is_empty() {
                     frame[idx] = (' ', (cell.fg[0], cell.fg[1], cell.fg[2]), (cell.bg[0], cell.bg[1], cell.bg[2]));
                 } else {

src/render/vulkan.rs

@@ -6,7 +6,6 @@ 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;
@@ -84,7 +83,6 @@ pub struct VulkanRenderer {
     descriptor_pool: vk::DescriptorPool,
     descriptor_set: vk::DescriptorSet,
     descriptor_set_layout: vk::DescriptorSetLayout,
-    tick_count: u64,
     window: Arc<winit::window::Window>,
 }
 
@@ -157,14 +155,12 @@ impl VulkanRenderer {
             atlas_image, atlas_memory, atlas_view, atlas_sampler, atlas_map,
             instance_buffer, instance_memory, instance_ptr, instance_count,
             descriptor_pool, descriptor_set, descriptor_set_layout,
-            tick_count: 0,
             window,
         })
     }
 
     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), (char, [u8; 4])> = std::collections::HashMap::new();
         for e in entities.all() {
@@ -306,7 +302,6 @@ impl VulkanRenderer {
         }
 
         self.frame_index = (self.frame_index + 1) % MAX_FRAMES;
-        self.tick_count += 1;
     }
 
     pub fn grid_w(&self) -> usize { self.grid_w }

src/render/window_input.rs

@@ -74,11 +74,4 @@ impl WindowInput {
         else if keys.contains(&KeyCode::Digit0) { self.paint = Some(0); }
         else if keys.contains(&KeyCode::KeyX) { self.paint = Some(99); }
     }
-
-    pub fn clear(&mut self) {
-        self.down_keys.clear();
-        self.left = false;
-        self.right = false;
-        self.jump = false;
-    }
 }