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, 12 insertions, 8 deletions

diff --git a/cnn_v3/shaders/gbuf_view.wgsl b/cnn_v3/shaders/gbuf_view.wgsl
index 3e7d1ff..d53b6f6 100644
--- a/cnn_v3/shaders/gbuf_view.wgsl
+++ b/cnn_v3/shaders/gbuf_view.wgsl
@@ -1,5 +1,5 @@
-// G-buffer channel visualization — 4×5 grid of 20 feature channels.
-// Takes feat_tex0 (rgba32uint, ch 0-7 f16) and feat_tex1 (rgba32uint, ch 8-19 unorm8).
+// G-buffer channel visualization — 4×6 grid of 23 feature channels.
+// Takes feat_tex0 (rgba32uint, ch 0-7 f16) and feat_tex1 (rgba32uint, ch 8-22 unorm8).
 // Outputs tiled channel view to a standard rgba8unorm render target.
 //
 // Channel layout (row×col):
@@ -7,7 +7,8 @@
 //   Row 1: ch4(nrm.y)  ch5(depth)  ch6(dzdx)   ch7(dzdy)
 //   Row 2: ch8(matid)  ch9(prv.r)  ch10(prv.g) ch11(prv.b)
 //   Row 3: ch12(m1.r)  ch13(m1.g)  ch14(m1.b)  ch15(m2.r)
-//   Row 4: ch16(m2.g)  ch17(m2.b)  ch18(shdw)  ch19(trns)
+//   Row 4: ch16(m2.g)  ch17(m2.b)  ch18(trns)  ch19(spare)
+//   Row 5: ch20(dif.r) ch21(dif.g) ch22(dif.b) ch23(spare)
 
 #include "debug/debug_print"
 
@@ -29,12 +30,12 @@ fn fs_main(@builtin(position) pos: vec4f) -> @location(0) vec4f {
     let uv  = pos.xy / u.resolution;
 
     let COLS = 4.0;
-    let ROWS = 5.0;
+    let ROWS = 6.0;
     let col  = u32(uv.x * COLS);
     let row  = u32(uv.y * ROWS);
     let ch   = row * 4u + col;
 
-    if (col >= 4u || ch >= 20u) {
+    if (col >= 4u || ch == 19u || ch >= 23u) {
         return vec4f(0.05, 0.05, 0.05, 1.0);
     }
 
@@ -71,7 +72,8 @@ fn fs_main(@builtin(position) pos: vec4f) -> @location(0) vec4f {
         var bytes: vec4f;
         if      (comp_idx == 0u) { bytes = unpack4x8unorm(t.x); }
         else if (comp_idx == 1u) { bytes = unpack4x8unorm(t.y); }
-        else                     { bytes = unpack4x8unorm(t.z); }
+        else if (comp_idx == 2u) { bytes = unpack4x8unorm(t.z); }
+        else                     { bytes = unpack4x8unorm(t.w); }
         var ba = array<f32, 4>(bytes.x, bytes.y, bytes.z, bytes.w);
         v = ba[sub];
     }
@@ -120,8 +122,10 @@ fn fs_main(@builtin(position) pos: vec4f) -> @location(0) vec4f {
         case 15u: { out = debug_str(out, pos.xy, origin, vec4u(0x6D322E72u, 0u, 0u, 0u), 4u); }          // m2.r
         case 16u: { out = debug_str(out, pos.xy, origin, vec4u(0x6D322E67u, 0u, 0u, 0u), 4u); }          // m2.g
         case 17u: { out = debug_str(out, pos.xy, origin, vec4u(0x6D322E62u, 0u, 0u, 0u), 4u); }          // m2.b
-        case 18u: { out = debug_str(out, pos.xy, origin, vec4u(0x73686477u, 0u, 0u, 0u), 4u); }          // shdw
-        default:  { out = debug_str(out, pos.xy, origin, vec4u(0x74726E73u, 0u, 0u, 0u), 4u); }          // trns
+        case 18u: { out = debug_str(out, pos.xy, origin, vec4u(0x74726E73u, 0u, 0u, 0u), 4u); }          // trns
+        case 20u: { out = debug_str(out, pos.xy, origin, vec4u(0x6469662Eu, 0x72000000u, 0u, 0u), 5u); } // dif.r
+        case 21u: { out = debug_str(out, pos.xy, origin, vec4u(0x6469662Eu, 0x67000000u, 0u, 0u), 5u); } // dif.g
+        default:  { out = debug_str(out, pos.xy, origin, vec4u(0x6469662Eu, 0x62000000u, 0u, 0u), 5u); } // dif.b
     }
     return out;
 }