feat(cnn_v3): Phase 4 — type-aware SDF in shadow pass

dfWithID() in gbuf_shadow.wgsl now branches on obj.params.x
(ObjectType) instead of using sdBox for everything:
  0=CUBE  → sdBox(lp, vec3(1))
  1=SPHERE → sdSphere(lp, 1.0)
  2=PLANE  → sdPlane(lp, vec3(0,1,0), obj.params.y)
  3=TORUS  → sdTorus(lp, vec2(0.8, 0.2))

36/36 tests pass.

2 files changed, 11 insertions, 4 deletions

diff --git a/cnn_v3/docs/HOWTO.md b/cnn_v3/docs/HOWTO.md
index 765d80b..5c5cc2a 100644
--- a/cnn_v3/docs/HOWTO.md
+++ b/cnn_v3/docs/HOWTO.md
@@ -106,7 +106,7 @@ outputs[1]  → feat_tex1   (rgba32uint: mat_id, prev.rgb, mip1.rgb, mip2.rgb, s
 | Internal scene + animation | ✅ Done | cubes + spheres + 2 lights |
 | Pass 2: SDF shadow | ✅ Done | `gbuf_shadow.wgsl`, proxy-box SDF per object |
 | Pass 3: Transparency | ❌ TODO | low priority, opaque scenes only |
-| Phase 4: type-aware SDF | ❌ TODO | optional refinement |
+| Phase 4: type-aware SDF | ✅ Done | switch on `obj.params.x` in `dfWithID` |
 
 ### Pass 2: SDF shadow raymarching
 
diff --git a/cnn_v3/shaders/gbuf_shadow.wgsl b/cnn_v3/shaders/gbuf_shadow.wgsl
index 36e7b28..0f5f8b4 100644
--- a/cnn_v3/shaders/gbuf_shadow.wgsl
+++ b/cnn_v3/shaders/gbuf_shadow.wgsl
@@ -35,9 +35,16 @@ fn dfWithID(p: vec3f) -> RayMarchResult {
 
     let n = u32(globals.params.x);
     for (var i = 0u; i < n; i++) {
-        let obj = object_data.objects[i];
-        let lp  = (obj.inv_model * vec4f(p, 1.0)).xyz;
-        let d   = sdBox(lp, vec3f(1.0));
+        let obj      = object_data.objects[i];
+        let lp       = (obj.inv_model * vec4f(p, 1.0)).xyz;
+        let obj_type = u32(obj.params.x);
+        var d: f32;
+        switch obj_type {
+            case 1u: { d = sdSphere(lp, 1.0); }                              // SPHERE
+            case 2u: { d = sdPlane(lp, vec3f(0.0, 1.0, 0.0), obj.params.y); } // PLANE
+            case 3u: { d = sdTorus(lp, vec2f(0.8, 0.2)); }                   // TORUS
+            default: { d = sdBox(lp, vec3f(1.0)); }                          // CUBE (0) + fallback
+        }
         if (d < res.distance) {
             res.distance  = d;
             res.object_id = f32(i + 1u);