wip(cnn_v3): shadow→dif intermediate + scene tweaks + migration plan

- gbuf_shadow.wgsl: normal bias 0.05→0.02
- gbuf_pack.wgsl: compute dif=diffuse*shadow, drop shadow from t1.z,
  store dif in t1.w (INTERMEDIATE — incorrect packing, see migration plan)
- gbuf_deferred.wgsl: read dif from t1.w.x (matches intermediate packing)
- gbuf_view.wgsl: expand to 4×6 grid, show dif.r/g/b in row 5
  (INTERMEDIATE — to be reverted to 4×5 with ch18=dif)
- gbuffer_effect.cc: add small hovering sphere (r=0.6) above scene;
  swap cube/sphere positions; both spheres pulsate
- docs/GBUF_DIF_MIGRATION.md: full migration plan with checklist

handoff(Claude): intermediate commit — GBUF_DIF_MIGRATION.md §Current State
describes what is wrong and the full implementation checklist (5 steps).

1 files changed, 4 insertions, 4 deletions

diff --git a/cnn_v3/shaders/gbuf_deferred.wgsl b/cnn_v3/shaders/gbuf_deferred.wgsl
index 2ed4ce3..bcc42cc 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[2] = pack4x8unorm(mip2.g, mip2.b, shadow, transp)
-    let t1     = textureLoad(feat_tex1, coord, 0);
-    let shadow = unpack4x8unorm(t1.z).z;
+    // feat_tex1[3] = pack4x8unorm(dif.r, dif.g, dif.b, spare) — dif = diffuse*shadow
+    let t1  = textureLoad(feat_tex1, coord, 0);
+    let dif = unpack4x8unorm(t1.w).x;
 
-    return vec4f(albedo * (AMBIENT + diffuse * shadow), 1.0);
+    return vec4f(albedo * (AMBIENT + dif), 1.0);
 }