feat(cnn_v3): add G-buffer visualizer + web sample loader (Phase 7)

C++ GBufViewEffect: renders all 20 feature channels from feat_tex0/feat_tex1
in a 4×5 tiled grid. Custom BGL with WGPUTextureSampleType_Uint; bind group
rebuilt per frame via wgpuRenderPipelineGetBindGroupLayout.

Web tool: "Load sample directory" button — webkitdirectory picker, FULL_PACK_SHADER
compute (matches gbuf_pack.wgsl packing), runFromFeat() skips photo-pack step,
computePSNR() readback + comparison vs target.png side-by-side.

36/36 tests pass. Docs updated: HOWTO.md §9, README, PROJECT_CONTEXT, TODO,
COMPLETED.

handoff(Gemini): CNN v3 Phase 7 done. Next: run train_cnn_v3.py (see HOWTO §3).

1 files changed, 5 insertions, 0 deletions

diff --git a/src/tests/gpu/test_demo_effects.cc b/src/tests/gpu/test_demo_effects.cc
index f5af5a9..d6e9c8a 100644
--- a/src/tests/gpu/test_demo_effects.cc
+++ b/src/tests/gpu/test_demo_effects.cc
@@ -89,6 +89,11 @@ static void test_effects() {
            fixture.ctx(),
            std::vector<std::string>{"gbuf_feat0", "gbuf_feat1"},
            std::vector<std::string>{"cnn_v3_output"}, 0.0f, 1000.0f)},
+      {"GBufViewEffect",
+       std::make_shared<GBufViewEffect>(
+           fixture.ctx(),
+           std::vector<std::string>{"gbuf_feat0", "gbuf_feat1"},
+           std::vector<std::string>{"gbuf_view_out"}, 0.0f, 1000.0f)},
   };
 
   int passed = 0;