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, 1 insertions, 1 deletions

diff --git a/cnn_v3/shaders/gbuf_shadow.wgsl b/cnn_v3/shaders/gbuf_shadow.wgsl
index 6c81d66..65ae1fa 100644
--- a/cnn_v3/shaders/gbuf_shadow.wgsl
+++ b/cnn_v3/shaders/gbuf_shadow.wgsl
@@ -121,7 +121,7 @@ fn fs_main(@builtin(position) pos: vec4f) -> @location(0) vec4f {
     // Use rasterized surface normal for bias — correct for sphere impostors.
     let nm       = textureLoad(normal_mat_tex, vec2i(pos.xy), 0);
     let nor      = oct_decode_unorm(nm.rg);
-    let bias_pos = world + nor * 0.05;
+    let bias_pos = world + nor * 0.02;
 
     // March shadow rays toward each light; take the darkest value.
     var shadow_val  = 1.0;