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, 35 insertions, 0 deletions

diff --git a/assets/final/shaders/circle_mask_render.wgsl b/assets/final/shaders/circle_mask_render.wgsl
new file mode 100644
index 0000000..902600e
--- /dev/null
+++ b/assets/final/shaders/circle_mask_render.wgsl
@@ -0,0 +1,35 @@
+// Circle mask render shader
+// Samples mask and draws green outside the circle
+
+@group(0) @binding(0) var mask_tex: texture_2d<f32>;
+@group(0) @binding(1) var mask_sampler: sampler;
+
+struct Uniforms {
+  width: f32,
+  height: f32,
+  _pad1: f32,
+  _pad2: f32,
+};
+
+@group(0) @binding(2) var<uniform> uniforms: Uniforms;
+
+struct VSOutput {
+  @builtin(position) position: vec4<f32>,
+};
+
+@vertex fn vs_main(@builtin(vertex_index) i: u32) -> VSOutput {
+  var pos = array<vec2<f32>, 3>(
+    vec2<f32>(-1, -1), vec2<f32>(3, -1), vec2<f32>(-1, 3));
+  return VSOutput(vec4<f32>(pos[i], 0.0, 1.0));
+}
+
+@fragment fn fs_main(@builtin(position) p: vec4<f32>) -> @location(0) vec4<f32> {
+  let uv = p.xy / vec2<f32>(uniforms.width, uniforms.height);
+  let mask_value = textureSample(mask_tex, mask_sampler, uv).r;
+
+  if (mask_value > 0.5) {
+    discard;
+  }
+
+  return vec4<f32>(0.0, 1.0, 0.0, 1.0);
+}