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, 7 insertions, 4 deletions

diff --git a/cnn_v3/shaders/gbuf_pack.wgsl b/cnn_v3/shaders/gbuf_pack.wgsl
index 333589c..dd8d73b 100644
--- a/cnn_v3/shaders/gbuf_pack.wgsl
+++ b/cnn_v3/shaders/gbuf_pack.wgsl
@@ -86,6 +86,9 @@ fn pack_features(@builtin(global_invocation_id) id: vec3u) {
     let mat_id_u8 = nm.b;                                       // mat_id already in [0,1]
     let shadow    = textureLoad(gbuf_shadow, coord, 0).r;
     let transp    = textureLoad(gbuf_transp, coord, 0).r;
+    let nor       = oct_decode_unorm(nm.rg);
+    let diffuse   = max(0.0, dot(nor, vec3f(0.408, 0.816, 0.408)));
+    let dif       = diffuse * shadow;
     let prev      = textureSampleLevel(prev_cnn, bilinear_sampler, uv, 0.0).rgb;
 
     // MIP 1: 2×2 box filter (half resolution context)
@@ -103,13 +106,13 @@ fn pack_features(@builtin(global_invocation_id) id: vec3u) {
     // Texture 1: 4 u32, each = pack4x8unorm of four u8 values
     // [0] mat_id | prev.r | prev.g | prev.b
     // [1] mip1.r | mip1.g | mip1.b | mip2.r
-    // [2] mip2.g | mip2.b | shadow | transp
-    // [3] spare  (0)
+    // [2] mip2.g | mip2.b | transp | (spare)
+    // [3] dif.r | dif.g | dif.b | (spare)  — dif = diffuse*shadow (scalar, stored in all 3)
     let t1 = vec4u(
         pack4x8unorm(vec4f(mat_id_u8, prev.r, prev.g, prev.b)),
         pack4x8unorm(vec4f(mip1.r, mip1.g, mip1.b, mip2.r)),
-        pack4x8unorm(vec4f(mip2.g, mip2.b, shadow, transp)),
-        0u
+        pack4x8unorm(vec4f(mip2.g, mip2.b, transp, 0.0)),
+        pack4x8unorm(vec4f(dif, dif, dif, 0.0))
     );
     textureStore(feat_tex1, coord, t1);
 }