feat(gpu): Add GPU procedural texture generation system

Phase 1 implementation complete:
- GPU compute shader for noise generation (gen_noise.wgsl)
- TextureManager extensions: create_gpu_noise_texture(), dispatch_noise_compute()
- Asset packer PROC_GPU() syntax support with validation
- ShaderComposer integration for #include resolution
- Zero CPU memory overhead (GPU-only textures)
- Init-time and on-demand generation modes

Technical details:
- 8×8 workgroup size for 256×256 textures
- UniformBuffer for params (width, height, seed, frequency)
- Storage texture binding (rgba8unorm, write-only)
- Lazy pipeline compilation on first use
- ~300 bytes code (Phase 1)

Testing:
- New test: test_gpu_procedural.cc (passes)
- All 34 tests passing (100%)

Future phases:
- Phase 2: Add gen_perlin, gen_grid compute shaders
- Phase 3: Variable dimensions, async generation

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

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