test(gpu): Add post-process helper utilities testing (Phase 2.2)

Created test_post_process_helper.cc to validate pipeline and bind group utilities:
- Tests create_post_process_pipeline() function
  - Validates shader module creation
  - Verifies bind group layout (3 bindings: sampler, texture, uniform)
  - Confirms render pipeline creation with standard topology
- Tests pp_update_bind_group() function
  - Creates bind groups with correct sampler/texture/uniform bindings
  - Validates bind group update/replacement (releases old, creates new)
- Full integration test
  - Combines pipeline + bind group setup
  - Executes complete render pass with post-process effect
  - Validates no WebGPU validation errors during rendering

Test infrastructure additions:
- Helper functions for creating post-process textures with TEXTURE_BINDING usage
- Helper for creating texture views
- Minimal valid post-process shader for smoke testing
- Uses gpu_init_color_attachment() for proper depthSlice handling (macOS)

Key technical details:
- Post-process textures require RENDER_ATTACHMENT + TEXTURE_BINDING + COPY_SRC usage
- Bind group layout: binding 0 (sampler), binding 1 (texture), binding 2 (uniform buffer)
- Render passes need depthSlice = WGPU_DEPTH_SLICE_UNDEFINED on non-Windows platforms

Added CMake target with dependencies:
- Links against gpu, 3d, audio, procedural, util libraries
- Minimal dependencies (no timeline/music generation needed)

Coverage: Validates core post-processing infrastructure used by all post-process effects
Zero binary size impact: All test code under #if !defined(STRIP_ALL)

Part of GPU Effects Test Infrastructure (Phase 2/3)
Phase 2 Complete: Effect classes + helper utilities tested
Next: Phase 3 (optional) - Individual effect render validation

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