fix: code review cleanup — bugs, dead code, factorization (-167 lines)

Bugs:
- B1: fix dead tempo debug (prev_tempo captured after assignment)
- B2: fix ReloadAssetsFromFile leak for disk-loaded assets; simplify DropAsset
- B3: fix get_free_pool_slot leak (unregister synth + free data on reuse)
- B4: volatile -> std::atomic with acquire/release in miniaudio_backend, synth
- B5: fix unaligned reads in scene_loader (memcpy-based read_f32/read_u32)
- B6: fix shader module + BGL + pipeline layout leaks in gpu.cc, pipeline_builder

Dead code:
- D1: remove unused particle_defs.h
- D3: remove create_post_process_pipeline_simple (zero callers)
- D4: remove empty gpu_draw()
- D5: remove write-only Hybrid3D::initialized_
- D6: remove legacy pending buffer path in audio.cc

Factorization:
- F1: Effect::run_fullscreen_pass() replaces boilerplate in 5 effects
- F2: particle_common.wgsl snippet, #include in 3 WGSL shaders
- F3: gpu_create_shader_module() helper, used in 3 call sites
- F5: get_world_aabb() shared between bvh.cc and physics.cc
- F6: samples_to_seconds() replaces 6 inline expressions
- F7: gpu_create_linear/nearest_sampler use SamplerCache; add nearest() preset

37/37 tests passing.

handoff(Claude): code review batch — all items verified, no regressions.

1 files changed, 30 insertions, 41 deletions

diff --git a/src/gpu/gpu.cc b/src/gpu/gpu.cc
index 0be5afe..f5f1515 100644
--- a/src/gpu/gpu.cc
+++ b/src/gpu/gpu.cc
@@ -5,6 +5,7 @@
 #include "gpu.h"
 #include "effects/shaders.h"
 #include "generated/timeline.h"
+#include "gpu/sampler_cache.h"
 #include "gpu/shader_composer.h"
 #include "platform/platform.h"
 
@@ -124,26 +125,28 @@ WGPUTextureView gpu_create_texture_view_2d(WGPUTexture texture,
   return wgpuTextureCreateView(texture, &view_desc);
 }
 
+WGPUShaderModule gpu_create_shader_module(WGPUDevice device,
+                                          const char* wgsl_code,
+                                          const char* label) {
+  WGPUShaderSourceWGSL wgsl_src = {};
+  wgsl_src.chain.sType = WGPUSType_ShaderSourceWGSL;
+  wgsl_src.code = str_view(wgsl_code);
+  WGPUShaderModuleDescriptor desc = {};
+  desc.label = label_view(label);
+  desc.nextInChain = &wgsl_src.chain;
+  return wgpuDeviceCreateShaderModule(device, &desc);
+}
+
 WGPUSampler gpu_create_linear_sampler(WGPUDevice device) {
-  WGPUSamplerDescriptor desc = {};
-  desc.addressModeU = WGPUAddressMode_ClampToEdge;
-  desc.addressModeV = WGPUAddressMode_ClampToEdge;
-  desc.addressModeW = WGPUAddressMode_ClampToEdge;
-  desc.magFilter = WGPUFilterMode_Linear;
-  desc.minFilter = WGPUFilterMode_Linear;
-  desc.mipmapFilter = WGPUMipmapFilterMode_Nearest;
-  desc.maxAnisotropy = 1;
-  return wgpuDeviceCreateSampler(device, &desc);
+  WGPUSampler s = SamplerCache::Get().get_or_create(device, SamplerCache::clamp());
+  if (s) wgpuSamplerAddRef(s); // Caller owns a reference (for RAII wrappers)
+  return s;
 }
 
 WGPUSampler gpu_create_nearest_sampler(WGPUDevice device) {
-  WGPUSamplerDescriptor desc = {};
-  desc.addressModeU = WGPUAddressMode_ClampToEdge;
-  desc.addressModeV = WGPUAddressMode_ClampToEdge;
-  desc.magFilter = WGPUFilterMode_Nearest;
-  desc.minFilter = WGPUFilterMode_Nearest;
-  desc.maxAnisotropy = 1;
-  return wgpuDeviceCreateSampler(device, &desc);
+  WGPUSampler s = SamplerCache::Get().get_or_create(device, SamplerCache::nearest());
+  if (s) wgpuSamplerAddRef(s); // Caller owns a reference (for RAII wrappers)
+  return s;
 }
 
 TextureWithView gpu_create_dummy_scene_texture(WGPUDevice device) {
@@ -167,15 +170,8 @@ RenderPass gpu_create_render_pass(WGPUDevice device, WGPUTextureFormat format,
   // Compose shader to resolve #include directives
   std::string composed_shader = ShaderComposer::Get().Compose({}, shader_code);
 
-  // Create Shader Module
-  WGPUShaderSourceWGSL wgsl_src = {};
-  wgsl_src.chain.sType = WGPUSType_ShaderSourceWGSL;
-  wgsl_src.code = str_view(composed_shader.c_str());
-  WGPUShaderModuleDescriptor shader_desc = {};
-  shader_desc.label = label_view("render_shader");
-  shader_desc.nextInChain = &wgsl_src.chain;
   WGPUShaderModule shader_module =
-      wgpuDeviceCreateShaderModule(device, &shader_desc);
+      gpu_create_shader_module(device, composed_shader.c_str(), "render_shader");
 
   // Create Bind Group Layout & Bind Group
   std::vector<WGPUBindGroupLayoutEntry> bgl_entries;
@@ -257,6 +253,10 @@ RenderPass gpu_create_render_pass(WGPUDevice device, WGPUTextureFormat format,
 
   pass.pipeline = wgpuDeviceCreateRenderPipeline(device, &pipeline_desc);
 
+  wgpuShaderModuleRelease(shader_module);
+  wgpuPipelineLayoutRelease(pipeline_layout);
+  wgpuBindGroupLayoutRelease(bind_group_layout);
+
   return pass;
 }
 
@@ -268,14 +268,8 @@ ComputePass gpu_create_compute_pass(WGPUDevice device, const char* shader_code,
   // Compose shader to resolve #include directives
   std::string composed_shader = ShaderComposer::Get().Compose({}, shader_code);
 
-  WGPUShaderSourceWGSL wgsl_src = {};
-  wgsl_src.chain.sType = WGPUSType_ShaderSourceWGSL;
-  wgsl_src.code = str_view(composed_shader.c_str());
-  WGPUShaderModuleDescriptor shader_desc = {};
-  shader_desc.label = label_view("compute_shader");
-  shader_desc.nextInChain = &wgsl_src.chain;
   WGPUShaderModule shader_module =
-      wgpuDeviceCreateShaderModule(device, &shader_desc);
+      gpu_create_shader_module(device, composed_shader.c_str(), "compute_shader");
 
   std::vector<WGPUBindGroupLayoutEntry> bgl_entries;
   std::vector<WGPUBindGroupEntry> bg_entries;
@@ -319,6 +313,11 @@ ComputePass gpu_create_compute_pass(WGPUDevice device, const char* shader_code,
   pipeline_desc.compute.entryPoint = str_view("main");
 
   pass.pipeline = wgpuDeviceCreateComputePipeline(device, &pipeline_desc);
+
+  wgpuShaderModuleRelease(shader_module);
+  wgpuPipelineLayoutRelease(pipeline_layout);
+  wgpuBindGroupLayoutRelease(bind_group_layout);
+
   return pass;
 }
 
@@ -439,16 +438,6 @@ void gpu_init(PlatformState* platform_state) {
 
 }
 
-void gpu_draw(float audio_peak, float aspect_ratio, float time, float beat_time,
-              float beat_phase) {
-  // Rendering is driven externally via the sequence pipeline
-  (void)audio_peak;
-  (void)aspect_ratio;
-  (void)time;
-  (void)beat_time;
-  (void)beat_phase;
-}
-
 void gpu_resize(int width, int height) {
   if (width <= 0 || height <= 0)
     return;