feat(gpu): Phase 2 - Add gen_perlin and gen_grid GPU compute shaders

Complete Phase 2 implementation:
- gen_perlin.wgsl: FBM with configurable octaves, amplitude decay
- gen_grid.wgsl: Grid pattern with configurable spacing/thickness
- TextureManager extensions: create_gpu_perlin_texture(), create_gpu_grid_texture()
- Asset packer now validates gen_noise, gen_perlin, gen_grid for PROC_GPU()
- 3 compute pipelines (lazy-init on first use)

Shader parameters:
- gen_perlin: seed, frequency, amplitude, amplitude_decay, octaves (32 bytes)
- gen_grid: width, height, grid_size, thickness (16 bytes)

test_3d_render migration:
- Replaced CPU sky texture (gen_perlin) with GPU version
- Replaced CPU noise texture (gen_noise) with GPU version
- Added new GPU grid texture (256x256, 32px grid, 2px lines)

Size impact:
- gen_perlin.wgsl: ~200 bytes (compressed)
- gen_grid.wgsl: ~100 bytes (compressed)
- Total Phase 2 code: ~300 bytes
- Cumulative (Phase 1+2): ~600 bytes

Testing:
- All 34 tests passing (100%)
- test_gpu_procedural validates all generators
- test_3d_render uses 3 GPU textures (noise, perlin, grid)

Next: Phase 3 - Variable dimensions, async generation, pipeline caching

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

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