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, 13 insertions, 3 deletions

diff --git a/cnn_v3/src/gbuffer_effect.cc b/cnn_v3/src/gbuffer_effect.cc
index 829c199..25fef4c 100644
--- a/cnn_v3/src/gbuffer_effect.cc
+++ b/cnn_v3/src/gbuffer_effect.cc
@@ -116,7 +116,7 @@ void GBufferEffect::set_scene() {
   // 2 large static cubes for shadow debugging.
   {
     Object3D obj(ObjectType::CUBE);
-    obj.position = vec3(-1.0f, 0.0f, 0.0f);
+    obj.position = vec3(1.0f, 0.0f, 0.0f);
     obj.scale    = vec3(0.6f, 0.6f, 0.6f);
     obj.color    = vec4(0.9f, 0.5f, 0.3f, 1.0f);
     scene_.add_object(obj);
@@ -124,7 +124,7 @@ void GBufferEffect::set_scene() {
   }
   {
     Object3D obj(ObjectType::SPHERE);
-    obj.position = vec3(1.0f, 0.0f, 0.0f);
+    obj.position = vec3(-1.0f, 0.0f, 0.0f);
     const float r = 0.9f;
     obj.scale    = vec3(r, r, r);
     obj.color    = vec4(0.3f, 0.6f, 0.9f, 1.0f);
@@ -133,7 +133,17 @@ void GBufferEffect::set_scene() {
     sphere_anims_.push_back({idx, r});
   }
 
-  // (sphere removed for shadow debugging)
+  // Second sphere: small, hovering above both objects, pulsating.
+  {
+    Object3D obj(ObjectType::SPHERE);
+    obj.position = vec3(0.0f, 2.2f, 0.0f);
+    const float r = 0.6f;
+    obj.scale    = vec3(r, r, r);
+    obj.color    = vec4(0.9f, 0.8f, 0.2f, 1.0f);
+    const int idx = (int)scene_.objects.size();
+    scene_.add_object(obj);
+    sphere_anims_.push_back({idx, r});
+  }
 
   // Camera: above and in front of the scene, looking at origin.
   camera_.set_look_at(vec3(0.0f, 2.5f, 6.0f),