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, 1 deletions

diff --git a/cnn_v3/README.md b/cnn_v3/README.md
index a22d823..f161bf4 100644
--- a/cnn_v3/README.md
+++ b/cnn_v3/README.md
@@ -31,7 +31,9 @@ Add images directly to these directories and commit them.
 
 ## Status
 
-**Design phase.** Architecture defined, G-buffer prerequisite pending.
+**Phase 1 complete.** G-buffer integrated (raster + pack), 35/35 tests pass.
+Training infrastructure ready. U-Net WGSL shaders are next.
 
+See `cnn_v3/docs/HOWTO.md` for the practical playbook.
 See `cnn_v3/docs/CNN_V3.md` for full design.
 See `cnn_v2/` for reference implementation.