refactor: centralize platform-specific code in gpu.h

Move platform-specific type definitions to gpu.h and establish coding rule
that platform ifdefs must be confined to gpu/platform layers.

- gpu.h: add GpuTextureCopyInfo, GpuTextureDataLayout type aliases
- effect.cc: use GpuTextureCopyInfo instead of platform ifdefs
- texture_manager.cc: use type aliases and label_view() helper
- CODING_STYLE.md: add platform-specific code section with rule

Tests: 34/34 passing

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

4 files changed, 50 insertions, 26 deletions

diff --git a/doc/CODING_STYLE.md b/doc/CODING_STYLE.md
index 5c86409..254a1c3 100644
--- a/doc/CODING_STYLE.md
+++ b/doc/CODING_STYLE.md
@@ -52,6 +52,41 @@ Always use `defined()` and closing comment.
 
 ---
 
+## Platform-Specific Code
+
+**Rule:** Platform-specific compilation (`#ifdef DEMO_CROSS_COMPILE_WIN32`, etc.) must be confined to `src/gpu/gpu.{h,cc}` and `src/platform/platform.{h,cc}`.
+
+### Correct
+```cpp
+// In gpu.h - abstract platform differences
+#if defined(DEMO_CROSS_COMPILE_WIN32)
+using GpuTextureCopyInfo = WGPUImageCopyTexture;
+#else
+using GpuTextureCopyInfo = WGPUTexelCopyTextureInfo;
+#endif
+
+// In effect.cc - use abstraction
+GpuTextureCopyInfo src_copy = {
+    .texture = src, .mipLevel = 0, .origin = {0, 0, 0}};
+```
+
+### Wrong
+```cpp
+// In effect.cc - direct platform check (FORBIDDEN)
+#if defined(DEMO_CROSS_COMPILE_WIN32)
+  WGPUImageCopyTexture src_copy = {...};
+#else
+  WGPUTexelCopyTextureInfo src_copy = {...};
+#endif
+```
+
+**Rationale:**
+- Centralizes platform handling in gpu/platform layers
+- Prevents #ifdef proliferation across codebase
+- Makes cross-platform testing easier
+
+---
+
 ## Struct Initialization
 
 ### Good
diff --git a/src/gpu/effect.cc b/src/gpu/effect.cc
index 117ede2..0e53862 100644
--- a/src/gpu/effect.cc
+++ b/src/gpu/effect.cc
@@ -44,17 +44,10 @@ void Effect::blit_input_to_output(WGPUCommandEncoder encoder,
     return;
   }
 
-#if defined(DEMO_CROSS_COMPILE_WIN32)
-  WGPUImageCopyTexture src_copy = {
+  GpuTextureCopyInfo src_copy = {
       .texture = src, .mipLevel = 0, .origin = {0, 0, 0}};
-  WGPUImageCopyTexture dst_copy = {
+  GpuTextureCopyInfo dst_copy = {
       .texture = dst, .mipLevel = 0, .origin = {0, 0, 0}};
-#else
-  WGPUTexelCopyTextureInfo src_copy = {
-      .texture = src, .mipLevel = 0, .origin = {0, 0, 0}};
-  WGPUTexelCopyTextureInfo dst_copy = {
-      .texture = dst, .mipLevel = 0, .origin = {0, 0, 0}};
-#endif
 
   WGPUExtent3D extent = {static_cast<unsigned int>(width_),
                          static_cast<unsigned int>(height_), 1};
diff --git a/src/gpu/gpu.h b/src/gpu/gpu.h
index 41eeb29..5e928b2 100644
--- a/src/gpu/gpu.h
+++ b/src/gpu/gpu.h
@@ -75,6 +75,15 @@ struct TextureWithView {
   WGPUTextureView view;
 };
 
+// Platform-abstracted texture copy types
+#if defined(DEMO_CROSS_COMPILE_WIN32)
+using GpuTextureCopyInfo = WGPUImageCopyTexture;
+using GpuTextureDataLayout = WGPUTextureDataLayout;
+#else
+using GpuTextureCopyInfo = WGPUTexelCopyTextureInfo;
+using GpuTextureDataLayout = WGPUTexelCopyBufferLayout;
+#endif
+
 GpuBuffer gpu_create_buffer(WGPUDevice device, size_t size, uint32_t usage,
                             const void* data = nullptr);
 TextureWithView gpu_create_texture_2d(WGPUDevice device, uint32_t width,
diff --git a/src/gpu/texture_manager.cc b/src/gpu/texture_manager.cc
index 5bca95b..bdeb508 100644
--- a/src/gpu/texture_manager.cc
+++ b/src/gpu/texture_manager.cc
@@ -2,22 +2,13 @@
 // It implements the TextureManager.
 
 #include "gpu/texture_manager.h"
+#include "gpu/gpu.h"
 #include "gpu/shader_composer.h"
 #include "platform/platform.h"
 #include <cstdio>
 #include <cstring>
 #include <vector>
 
-#if defined(DEMO_CROSS_COMPILE_WIN32)
-// Old API
-#define WGPU_TEX_COPY_INFO WGPUImageCopyTexture
-#define WGPU_TEX_DATA_LAYOUT WGPUTextureDataLayout
-#else
-// New API
-#define WGPU_TEX_COPY_INFO WGPUTexelCopyTextureInfo
-#define WGPU_TEX_DATA_LAYOUT WGPUTexelCopyBufferLayout
-#endif
-
 void TextureManager::init(WGPUDevice device, WGPUQueue queue) {
   device_ = device;
   queue_ = queue;
@@ -79,22 +70,18 @@ void TextureManager::create_texture(const std::string& name, int width,
   tex_desc.format = WGPUTextureFormat_RGBA8Unorm;
   tex_desc.mipLevelCount = 1;
   tex_desc.sampleCount = 1;
-#if defined(DEMO_CROSS_COMPILE_WIN32)
-  tex_desc.label = nullptr;
-#else
-  tex_desc.label = {nullptr, 0};
-#endif
+  tex_desc.label = label_view(nullptr);
 
   WGPUTexture texture = wgpuDeviceCreateTexture(device_, &tex_desc);
 
   // 3. Upload Data
-  WGPU_TEX_COPY_INFO destination = {};
+  GpuTextureCopyInfo destination = {};
   destination.texture = texture;
   destination.mipLevel = 0;
   destination.origin = {0, 0, 0};
   destination.aspect = WGPUTextureAspect_All;
 
-  WGPU_TEX_DATA_LAYOUT source_layout = {};
+  GpuTextureDataLayout source_layout = {};
   source_layout.offset = 0;
   source_layout.bytesPerRow = width * 4;
   source_layout.rowsPerImage = height;