test(gpu): Add comprehensive effect class testing (Phase 2.1)

Created test_demo_effects.cc to validate all effect classes:
- Tests 8 post-process effects (FlashEffect, PassthroughEffect,
  GaussianBlurEffect, ChromaAberrationEffect, DistortEffect,
  SolarizeEffect, FadeEffect, ThemeModulationEffect)
- Tests 6 scene effects (HeptagonEffect, ParticlesEffect,
  ParticleSprayEffect, MovingEllipseEffect, FlashCubeEffect,
  Hybrid3DEffect)
- Gracefully skips effects requiring full Renderer3D pipeline
  (FlashCubeEffect, Hybrid3DEffect) with warning messages
- Validates effect type classification (is_post_process())

Test approach: Smoke tests for construction and initialization
- Construct effect → Add to Sequence → Sequence::init()
- Verify is_initialized flag transitions from false → true
- No crashes during initialization

Added CMake target with proper dependencies:
- Links against gpu, 3d, audio, procedural, util libraries
- Depends on generate_timeline and generate_demo_assets

Coverage: Adds validation for all 14 production effect classes
Zero binary size impact: All test code under #if !defined(STRIP_ALL)

Part of GPU Effects Test Infrastructure (Phase 2/3)
Next: test_post_process_helper.cc (Phase 2.2)

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