feat: Fix CircleMaskEffect and RotatingCubeEffect auxiliary texture masking

Resolves critical shader composition and format mismatch issues, enabling the
circle mask + rotating cube demonstration at 2.0s-4.0s in the timeline.

**Key Fixes:**

1. **Shader Function Name** (masked_cube.wgsl):
   - Fixed call to `ray_box_intersection()` (was incorrectly `ray_box()`)
   - Updated return value handling to use `RayBounds` struct (`.hit`, `.t_entry`, `.t_exit`)
   - Removed unused `sky_tex` binding to match pipeline layout expectations (5 bindings → 4)

2. **Shader Lifetime Issue** (rotating_cube_effect.h/cc):
   - Added `std::string composed_shader_` member to persist shader source
   - Prevents use-after-free when WebGPU asynchronously parses shader code

3. **Format Mismatch** (circle_mask_effect.cc):
   - Changed compute pipeline format from hardcoded `RGBA8Unorm` to `ctx_.format` (Bgra8UnormSrgb)
   - Matches auxiliary texture format created by `MainSequence::register_auxiliary_texture()`
   - Added depth stencil state to render pipeline to match scene pass requirements:
     * Format: Depth24Plus
     * depthWriteEnabled: False (no depth writes needed)
     * depthCompare: Always (always pass depth test)

4. **WebGPU Descriptor Initialization** (circle_mask_effect.cc):
   - Added `depthSlice = WGPU_DEPTH_SLICE_UNDEFINED` for non-Windows builds
   - Ensures proper initialization of render pass color attachments

5. **Test Coverage** (test_demo_effects.cc):
   - Updated EXPECTED_SCENE_COUNT: 6 → 8 (added CircleMaskEffect, RotatingCubeEffect)
   - Marked both effects as requiring 3D pipeline setup (skipped in basic tests)

**Technical Details:**

- **Auxiliary Texture Flow**: CircleMaskEffect generates mask (1.0 inside, 0.0 outside) →
  RotatingCubeEffect samples mask to render only inside circle → GaussianBlurEffect post-processes

- **Pipeline Format Matching**: All pipelines targeting same render pass must use matching formats:
  * Color: Bgra8UnormSrgb (system framebuffer format)
  * Depth: Depth24Plus (scene pass depth buffer)

- **ShaderComposer Integration**: Relies on `InitShaderComposer()` (called in `gpu.cc:372`)
  registering snippets: `common_uniforms`, `math/sdf_utils`, `ray_box`, etc.

**Effect Behavior:**
- Runs from 2.0s to 4.0s in demo timeline
- CircleMaskEffect (priority 0): Draws green outside circle, transparent inside
- RotatingCubeEffect (priority 1): Renders bump-mapped cube inside circle
- GaussianBlurEffect (priority 2): Post-process blur on entire composition

**Test Results:**
- All 33 tests pass (100%)
- No WebGPU validation errors
- Demo runs cleanly with `--seek 2.0`

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

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