docs: archive stale/completed docs, compact active refs (-1300 lines)

- Archive WORKSPACE_SYSTEM.md (completed); replace with 36-line operational ref
- Archive SHADER_REUSE_INVESTIGATION.md (implemented Feb 2026)
- Archive GPU_PROCEDURAL_PHASE4.md (completed feature)
- Archive GEOM_BUFFER.md (ideation only, never implemented)
- Archive SPECTRAL_BRUSH_EDITOR.md (v1 DCT approach, superseded by MQ v2)
- Update CLAUDE.md Tier 3 refs; point Audio to SPECTRAL_BRUSH_2.md
- Update TODO.md Task #5 design link to SPECTRAL_BRUSH_2.md
- Update COMPLETED.md archive index

handoff(Claude): doc cleanup done, 30 active docs (was 34), -1300 lines

1 files changed, 70 insertions, 0 deletions

diff --git a/doc/archive/GPU_PROCEDURAL_PHASE4.md b/doc/archive/GPU_PROCEDURAL_PHASE4.md
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+# GPU Procedural Phase 4: Texture Composition
+
+**Status:** ✅ Complete
+
+## Implementation
+
+Multi-input composite shaders with configurable sampler support.
+
+### API
+
+```cpp
+enum class SamplerType {
+  LinearClamp, LinearRepeat, NearestClamp, NearestRepeat
+};
+
+void create_gpu_composite_texture(
+    const std::string& name,
+    const std::string& shader_func,
+    const char* shader_code,
+    const void* uniform_data,
+    size_t uniform_size,
+    int width, int height,
+    const std::vector<std::string>& input_names,
+    SamplerType sampler = SamplerType::LinearClamp);
+```
+
+### Shaders
+
+**gen_blend.wgsl** - Blend two textures with lerp factor:
+- Bindings: output (0), uniform (1), input_a (2), input_b (3), sampler (4)
+- Uniform: `{u32 width, height; f32 blend_factor, _pad0}`
+
+**gen_mask.wgsl** - Multiply textures (masking):
+- Bindings: output (0), uniform (1), input_a (2), input_b (3), sampler (4)
+- Uniform: `{u32 width, height}`
+
+### Usage
+
+```cpp
+extern const char* gen_blend_compute_wgsl;
+
+struct { uint32_t width, height; float blend_factor, _pad0; } uni = {256, 256, 0.5f, 0.0f};
+
+tex_mgr.create_gpu_composite_texture(
+    "blended", "gen_blend", gen_blend_compute_wgsl,
+    &uni, sizeof(uni), 256, 256,
+    {"noise_a", "noise_b"},
+    SamplerType::LinearClamp);
+```
+
+### Features
+
+- **Dynamic bind groups:** N input textures + 1 sampler
+- **Lazy sampler creation:** Map-based cache, 4 preset types
+- **Multi-stage composition:** Composite of composites supported
+- **Guarded with `#if !defined(STRIP_GPU_COMPOSITE)`**
+
+### Size Impact
+
+- Code: ~460 lines added
+- Compressed: ~830 bytes (2 shaders + dispatch logic)
+
+### Tests
+
+`test_gpu_composite.cc`:
+- Blend two noise textures
+- Mask noise with grid
+- Multi-stage composite (composite of composites)
+
+All 35 tests passing.