feat(cnn_v3): shadow→dif migration complete (ch18)

Replace raw shadow (ch18) with dif = max(0,dot(normal,KEY_LIGHT))*shadow
across all layers. Channel count stays 20, weight shapes unchanged.

- gbuf_pack.wgsl: t1.z = pack4x8unorm(mip2.g, mip2.b, dif, transp); t1.w = 0u
- gbuf_deferred.wgsl: read dif from unpack4x8unorm(t1.z).z
- gbuf_view.wgsl: revert to 4×5 grid, ch18=dif label, ch19=trns label
- tools/shaders.js: FULL_PACK_SHADER adds oct_decode + computes dif
- cnn_v3_utils.py: assemble_features() computes dif on-the-fly via oct_decode
- docs: CNN_V3.md, HOWTO.md, HOW_TO_CNN.md, GBUF_DIF_MIGRATION.md updated

handoff(Gemini): shadow→dif migration done, ready for first training pass

1 files changed, 2 insertions, 2 deletions

diff --git a/cnn_v3/shaders/gbuf_deferred.wgsl b/cnn_v3/shaders/gbuf_deferred.wgsl
index bcc42cc..7257122 100644
--- a/cnn_v3/shaders/gbuf_deferred.wgsl
+++ b/cnn_v3/shaders/gbuf_deferred.wgsl
@@ -40,9 +40,9 @@ fn fs_main(@builtin(position) pos: vec4f) -> @location(0) vec4f {
     let normal  = oct_decode(vec2f(bx.y, ny_d.x));
     let diffuse = max(0.0, dot(normal, KEY_LIGHT));
 
-    // feat_tex1[3] = pack4x8unorm(dif.r, dif.g, dif.b, spare) — dif = diffuse*shadow
+    // feat_tex1[2] = pack4x8unorm(mip2.g, mip2.b, dif, transp) — dif at byte 2
     let t1  = textureLoad(feat_tex1, coord, 0);
-    let dif = unpack4x8unorm(t1.w).x;
+    let dif = unpack4x8unorm(t1.z).z;
 
     return vec4f(albedo * (AMBIENT + dif), 1.0);
 }