feat: Add CircleMaskEffect and RotatingCubeEffect with auxiliary texture masking

Implements demonstration of auxiliary texture masking system with:
- CircleMaskEffect: Generates circular mask, renders green outside circle
- RotatingCubeEffect: Renders SDF cube inside circle using mask

Architecture:
- Mask generation in compute phase (1.0 inside, 0.0 outside)
- CircleMask renders green where mask < 0.5 (outside circle)
- RotatingCube samples mask and discards where mask < 0.5

Files added:
- src/gpu/effects/circle_mask_effect.{h,cc}
- src/gpu/effects/rotating_cube_effect.{h,cc}
- assets/final/shaders/{circle_mask_compute,circle_mask_render,masked_cube}.wgsl

Build system:
- Updated CMakeLists.txt with new effect sources
- Registered shaders in demo_assets.txt
- Updated test_demo_effects.cc (8 scene effects total)

Status: Effects temporarily disabled in demo.seq (lines 31-35)
- ShaderComposer snippet registration needed for masked_cube.wgsl
- All 33 tests pass, demo runs without crashes

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

1 files changed, 51 insertions, 0 deletions

diff --git a/src/gpu/effects/rotating_cube_effect.h b/src/gpu/effects/rotating_cube_effect.h
new file mode 100644
index 0000000..1ce81b7
--- /dev/null
+++ b/src/gpu/effects/rotating_cube_effect.h
@@ -0,0 +1,51 @@
+// This file is part of the 64k demo project.
+// It defines RotatingCubeEffect for rendering a bump-mapped rotating cube.
+// Uses auxiliary texture masking to render only inside a circular region.
+
+#ifndef ROTATING_CUBE_EFFECT_H_
+#define ROTATING_CUBE_EFFECT_H_
+
+#include "gpu/effect.h"
+#include "gpu/gpu.h"
+#include "util/mini_math.h"
+
+class RotatingCubeEffect : public Effect {
+ public:
+  RotatingCubeEffect(const GpuContext& ctx);
+  ~RotatingCubeEffect() override;
+
+  void init(MainSequence* demo) override;
+  void render(WGPURenderPassEncoder pass, float time, float beat,
+              float intensity, float aspect_ratio) override;
+
+ private:
+  struct Uniforms {
+    mat4 view_proj;
+    mat4 inv_view_proj;
+    vec4 camera_pos_time;
+    vec4 params;
+    vec2 resolution;
+    vec2 padding;
+  };
+
+  struct ObjectData {
+    mat4 model;
+    mat4 inv_model;
+    vec4 color;
+    vec4 params;
+  };
+
+  MainSequence* demo_ = nullptr;
+  WGPURenderPipeline pipeline_ = nullptr;
+  WGPUBindGroup bind_group_0_ = nullptr;
+  WGPUBindGroup bind_group_1_ = nullptr;
+  GpuBuffer uniform_buffer_;
+  GpuBuffer object_buffer_;
+  WGPUTexture noise_texture_ = nullptr;
+  WGPUTextureView noise_view_ = nullptr;
+  WGPUSampler noise_sampler_ = nullptr;
+  WGPUSampler mask_sampler_ = nullptr;
+  float rotation_ = 0.0f;
+};
+
+#endif /* ROTATING_CUBE_EFFECT_H_ */