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, 50 insertions, 0 deletions

diff --git a/src/gpu/effects/circle_mask_effect.h b/src/gpu/effects/circle_mask_effect.h
new file mode 100644
index 0000000..57f2389
--- /dev/null
+++ b/src/gpu/effects/circle_mask_effect.h
@@ -0,0 +1,50 @@
+// This file is part of the 64k demo project.
+// It defines the CircleMaskEffect class for masking system demonstration.
+// Creates a circular mask and renders green outside the circle.
+
+#ifndef CIRCLE_MASK_EFFECT_H_
+#define CIRCLE_MASK_EFFECT_H_
+
+#include "gpu/effect.h"
+#include "gpu/uniform_helper.h"
+
+class CircleMaskEffect : public Effect {
+ public:
+  CircleMaskEffect(const GpuContext& ctx, float radius = 0.4f);
+  ~CircleMaskEffect() override;
+
+  void init(MainSequence* demo) override;
+  void compute(WGPUCommandEncoder encoder, float time, float beat,
+               float intensity, float aspect_ratio) override;
+  void render(WGPURenderPassEncoder pass, float time, float beat,
+              float intensity, float aspect_ratio) override;
+
+ private:
+  struct ComputeUniforms {
+    float radius;
+    float aspect_ratio;
+    float width;
+    float height;
+  };
+
+  struct RenderUniforms {
+    float width;
+    float height;
+    float _pad1;
+    float _pad2;
+  };
+
+  MainSequence* demo_ = nullptr;
+  float radius_;
+
+  WGPURenderPipeline compute_pipeline_ = nullptr;
+  WGPUBindGroup compute_bind_group_ = nullptr;
+  UniformBuffer<ComputeUniforms> compute_uniforms_;
+
+  WGPURenderPipeline render_pipeline_ = nullptr;
+  WGPUBindGroup render_bind_group_ = nullptr;
+  WGPUSampler mask_sampler_ = nullptr;
+  UniformBuffer<RenderUniforms> render_uniforms_;
+};
+
+#endif /* CIRCLE_MASK_EFFECT_H_ */