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, 8 insertions, 0 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 26818c3..fb6beef 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -601,6 +601,14 @@ if(DEMO_BUILD_TESTS)
     target_link_libraries(test_gpu_procedural PRIVATE 3d gpu audio procedural util ${DEMO_LIBS})
     add_dependencies(test_gpu_procedural generate_demo_assets)
 
+    # GPU Composite Texture Test (Phase 4)
+    add_demo_test(test_gpu_composite GpuCompositeTest
+        src/tests/test_gpu_composite.cc
+        ${PLATFORM_SOURCES}
+        ${GEN_DEMO_CC})
+    target_link_libraries(test_gpu_composite PRIVATE 3d gpu audio procedural util ${DEMO_LIBS})
+    add_dependencies(test_gpu_composite generate_demo_assets)
+
     # Gantt chart output test (bash script)
     add_test(
         NAME GanttOutputTest