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>

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