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, 2 insertions, 2 deletions

diff --git a/PROJECT_CONTEXT.md b/PROJECT_CONTEXT.md
index 6b4468f..cadd514 100644
--- a/PROJECT_CONTEXT.md
+++ b/PROJECT_CONTEXT.md
@@ -36,7 +36,7 @@
 - **Audio:** Sample-accurate sync. Zero heap allocations per frame. Variable tempo. OLA-IDCT synthesis (v2 .spec): Hann analysis window, rectangular synthesis, 50% overlap, click-free. V1 (raw DCT-512) preserved for generated notes. .spec files regenerated as v2.
 - **Shaders:** Parameterized effects (UniformHelper, .seq syntax). Beat-synchronized animation support (`beat_time`, `beat_phase`). Modular WGSL composition with ShaderComposer. 27 shared common shaders (math, render, compute). Reusable snippets: `render/scratch_lines`, `render/ntsc_common` (NTSC signal processing, RGB and YIQ input variants via `sample_ntsc_signal` hook), `math/color` (YIQ/NTSC), `math/color_c64` (C64 palette, Bayer dither, border animation).
 - **3D:** Hybrid SDF/rasterization with BVH. Binary scene loader. Blender pipeline.
-- **Effects:** CNN post-processing: CNNEffect (v1) and CNNv2Effect operational. CNN v2: sigmoid activation, storage buffer weights (~3.2 KB), 7D static features, dynamic layers. Training stable, convergence validated.
+- **Effects:** CNN post-processing: CNNEffect (v1) and CNNv2Effect operational. CNN v2: sigmoid activation, storage buffer weights (~3.2 KB), 7D static features, dynamic layers. Training stable, convergence validated. **CNN v3 Phase 1 complete:** `GBufferEffect` integrated (MRT raster + pack compute, 20-channel feature textures). See `cnn_v3/docs/HOWTO.md`.
 - **Tools:** CNN test tool operational. Texture readback utility functional. Timeline editor (web-based, beat-aligned, audio playback).
 - **Build:** Asset dependency tracking. Size measurement. Hot-reload (debug-only). WSL (Windows 10) supported: native Linux build and cross-compile to `.exe` via `mingw-w64`.
 - **Sequence:** DAG-based effect routing with explicit node system. Python compiler with topological sort and ping-pong optimization. 12 effects operational (Passthrough, Placeholder, GaussianBlur, Heptagon, Particles, RotatingCube, Hybrid3D, Flash, PeakMeter, Scene1, Scene2, Scratch). Effect times are absolute (seq_compiler adds sequence start offset). See `doc/SEQUENCE.md`.
@@ -46,7 +46,7 @@
 
 ## Next Up
 
-**Active:** Spectral Brush Editor (procedural compression), CNN v2 quantization
+**Active:** CNN v3 Phase 3 (WGSL U-Net shaders), Spectral Brush Editor
 **Ongoing:** Test infrastructure maintenance (35/35 passing)
 **Future:** Size optimization (64k target), 3D enhancements