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, 3 insertions, 0 deletions

diff --git a/src/gpu/demo_effects.h b/src/gpu/demo_effects.h
index 9d794aa..91ab6f2 100644
--- a/src/gpu/demo_effects.h
+++ b/src/gpu/demo_effects.h
@@ -32,6 +32,9 @@
 #include "effects/scratch_effect.h"
 #include "effects/ntsc_effect.h"
 
+// CNN v3 G-buffer
+#include "../../cnn_v3/src/gbuffer_effect.h"
+
 // TODO: Port CNN effects
 // #include "../../cnn_v1/src/cnn_v1_effect.h"
 // #include "../../cnn_v2/src/cnn_v2_effect.h"