feat: GPU procedural Phase 4 - texture composition

Multi-input composite shaders with sampler support.
- Dynamic bind group layouts (N input textures + 1 sampler)
- dispatch_composite() for multi-input compute dispatch
- create_gpu_composite_texture() API
- gen_blend.wgsl and gen_mask.wgsl shaders

Guarded with #if !defined(STRIP_GPU_COMPOSITE) for easy removal.

Tests:
- Blend two noise textures
- Mask noise with grid
- Multi-stage composite (composite of composites)

Size: ~830 bytes (2 shaders + dispatch logic)

handoff(Claude): GPU procedural Phase 4 complete

1 files changed, 2 insertions, 0 deletions

diff --git a/assets/final/demo_assets.txt b/assets/final/demo_assets.txt
index 0f7c267..819c9c9 100644
--- a/assets/final/demo_assets.txt
+++ b/assets/final/demo_assets.txt
@@ -55,6 +55,8 @@ SHADER_VIGNETTE, NONE, shaders/vignette.wgsl, "Vignette Shader"
 SHADER_COMPUTE_GEN_NOISE, NONE, shaders/compute/gen_noise.wgsl, "GPU Noise Compute Shader"
 SHADER_COMPUTE_GEN_PERLIN, NONE, shaders/compute/gen_perlin.wgsl, "GPU Perlin Noise Compute Shader"
 SHADER_COMPUTE_GEN_GRID, NONE, shaders/compute/gen_grid.wgsl, "GPU Grid Compute Shader"
+SHADER_COMPUTE_GEN_BLEND, NONE, shaders/compute/gen_blend.wgsl, "GPU Blend Composite Shader"
+SHADER_COMPUTE_GEN_MASK, NONE, shaders/compute/gen_mask.wgsl, "GPU Mask Composite Shader"
 CIRCLE_MASK_COMPUTE_SHADER, NONE, shaders/circle_mask_compute.wgsl, "Circle mask compute shader"
 CIRCLE_MASK_RENDER_SHADER, NONE, shaders/circle_mask_render.wgsl, "Circle mask render shader"
 MASKED_CUBE_SHADER, NONE, shaders/masked_cube.wgsl, "Masked cube shader"