feat(cnn_v3): Phase 1 complete - GBufferEffect integrated + HOWTO playbook

- Wire GBufferEffect into demo build: assets.txt, DemoSourceLists.cmake,
  demo_effects.h, shaders.h/cc. ShaderComposer::Compose() applied to
  gbuf_raster.wgsl (resolves #include "common_uniforms").
- Add GBufferEffect construction test. 35/35 passing.
- Write cnn_v3/docs/HOWTO.md: G-buffer wiring, training data prep,
  training plan, per-pixel validation workflow, phase status table,
  troubleshooting guide.
- Add project hooks: remind to update HOWTO.md on cnn_v3/ edits;
  warn on direct str_view(*_wgsl) usage bypassing ShaderComposer.
- Update PROJECT_CONTEXT.md and TODO.md: Phase 1 done,
  Phase 3 (WGSL U-Net shaders) is next active.

handoff(Gemini): CNN v3 Phase 3 is next - WGSL enc/dec/bottleneck/FiLM
shaders in cnn_v3/shaders/. See cnn_v3/docs/CNN_V3.md Architecture
section and cnn_v3/docs/HOWTO.md section 3 for spec. GBufferEffect
outputs feat_tex0 + feat_tex1 (rgba32uint, 20ch, 32 bytes/pixel).
C++ CNNv3Effect (Phase 4) takes those as input nodes.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/effects/shaders.h b/src/effects/shaders.h
index 527a8a3..cf095fb 100644
--- a/src/effects/shaders.h
+++ b/src/effects/shaders.h
@@ -20,6 +20,10 @@ extern const char* scratch_shader_wgsl;
 extern const char* ntsc_rgb_shader_wgsl;
 extern const char* ntsc_yiq_shader_wgsl;
 
+// CNN v3 G-buffer shaders
+extern const char* gbuf_raster_wgsl;
+extern const char* gbuf_pack_wgsl;
+
 // Compute shaders
 extern const char* gen_noise_compute_wgsl;
 extern const char* gen_perlin_compute_wgsl;