fix: use ShaderComposer in RotatingCube; add rule to CODING_STYLE

rotating_cube_effect.cc was bypassing ShaderComposer, causing #include
directives in rotating_cube.wgsl to fail at runtime.

handoff(Claude): ShaderComposer rule documented and enforced in rotating_cube.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

2 files changed, 21 insertions, 1 deletions

diff --git a/doc/CODING_STYLE.md b/doc/CODING_STYLE.md
index 7d45027..6ef7326 100644
--- a/doc/CODING_STYLE.md
+++ b/doc/CODING_STYLE.md
@@ -166,6 +166,24 @@ Three-line header for all source files.
 
 ## WGSL Shader Style
 
+### Always Use ShaderComposer (Required)
+
+**Rule:** Always load `.wgsl` shaders via `ShaderComposer::Get().Compose(...)`, even if the shader has no `#include` directives. Never pass the raw shader string directly to `wgpuDeviceCreateShaderModule`.
+
+```cpp
+// Correct
+const std::string src = ShaderComposer::Get().Compose({}, my_shader_wgsl);
+WGPUShaderSourceWGSL wgsl_src = {};
+wgsl_src.code = str_view(src);
+
+// Wrong - bypasses #include resolution
+wgsl_src.code = str_view(my_shader_wgsl);
+```
+
+**Rationale:** Shaders may gain `#include` directives later; bypassing `ShaderComposer` silently breaks them at runtime with no compile error.
+
+---
+
 ### File Header (Required)
 
 Every `.wgsl` file must start with a 2-line comment header:
diff --git a/src/effects/rotating_cube_effect.cc b/src/effects/rotating_cube_effect.cc
index 197bc26..6ee3e85 100644
--- a/src/effects/rotating_cube_effect.cc
+++ b/src/effects/rotating_cube_effect.cc
@@ -6,6 +6,7 @@
 #include "gpu/bind_group_builder.h"
 #include "gpu/gpu.h"
 #include "gpu/post_process_helper.h"
+#include "gpu/shader_composer.h"
 #include "util/fatal_error.h"
 
 RotatingCube::RotatingCube(const GpuContext& ctx,
@@ -43,9 +44,10 @@ RotatingCube::RotatingCube(const GpuContext& ctx,
       wgpuDeviceCreatePipelineLayout(ctx_.device, &pl_desc);
 
   // Load shader
+  const std::string composed = ShaderComposer::Get().Compose({}, rotating_cube_wgsl);
   WGPUShaderSourceWGSL wgsl_src = {};
   wgsl_src.chain.sType = WGPUSType_ShaderSourceWGSL;
-  wgsl_src.code = str_view(rotating_cube_wgsl);
+  wgsl_src.code = str_view(composed.c_str());
 
   WGPUShaderModuleDescriptor shader_desc = {};
   shader_desc.nextInChain = &wgsl_src.chain;