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/workspaces/main/assets.txt b/workspaces/main/assets.txt
index b50f2fb..ad57d2f 100644
--- a/workspaces/main/assets.txt
+++ b/workspaces/main/assets.txt
@@ -97,6 +97,10 @@ SHADER_SCRATCH, WGSL, ../../src/effects/scratch.wgsl, "Scratch effect shader"
 SHADER_NTSC_RGB, WGSL, ../../src/effects/ntsc_rgb.wgsl, "NTSC effect shader (RGB input)"
 SHADER_NTSC_YIQ, WGSL, ../../src/effects/ntsc_yiq.wgsl, "NTSC effect shader (YIQ input)"
 SHADER_RENDER_NTSC_COMMON, WGSL, ../../src/shaders/render/ntsc_common.wgsl, "NTSC shared constants and functions snippet"
+
+# --- CNN v3 G-Buffer ---
+SHADER_GBUF_RASTER, WGSL, ../../cnn_v3/shaders/gbuf_raster.wgsl, "CNN v3 G-buffer MRT rasterization shader"
+SHADER_GBUF_PACK, WGSL, ../../cnn_v3/shaders/gbuf_pack.wgsl, "CNN v3 G-buffer feature pack compute shader"
 SHADER_DEBUG_DEBUG_PRINT, WGSL, ../../src/shaders/debug/debug_print.wgsl, "Debug print snippet"
 
 # --- Sequence Shaders ---