fix: cube.obj winding order — all faces now CCW from outside

Root cause: ALL 12 faces in cube.obj had reversed winding order.
This caused every face normal to point inward, breaking:
- Diffuse lighting (NdotL < 0 → no light on any face)
- Shadow bias (slope-dependent bias used wrong NdotL)
- Shadow self-shadowing (both sides rendered with CULL_NONE)

Fix: reversed all face indices to correct CCW winding:
- Back face: f 1 4 3, f 1 3 2 (was f 1 2 3, f 1 3 4)
- Front face: f 5 6 7, f 5 7 8 (was f 5 7 6, f 5 8 7)
- Left/Right/Bottom/Top: all reversed

Now MeshMath.ComputeFaceNormal produces correct outward normals.
Normal flip in shader (dot(N,V) < 0) is still kept as safety for
double-sided rendering, but cubes now have correct normals natively.

Content/cube.obj

@@ -1,4 +1,4 @@
-# Simple unit cube
+# Simple unit cube — consistent CCW winding (viewed from outside)
 v -0.5 -0.5 -0.5
 v  0.5 -0.5 -0.5
 v  0.5  0.5 -0.5
@@ -8,26 +8,26 @@ v  0.5 -0.5  0.5
 v  0.5  0.5  0.5
 v -0.5  0.5  0.5
 
-# Back face
-f 1 2 3
-f 1 3 4
+# Back face (normal -Z, CCW from outside = looking from -Z)
+f 1 4 3
+f 1 3 2
 
-# Front face
-f 5 7 6
-f 5 8 7
+# Front face (normal +Z, CCW from outside = looking from +Z)
+f 5 6 7
+f 5 7 8
 
-# Left face
-f 1 5 6
-f 1 6 2
+# Left face (normal -X, CCW from outside = looking from -X)
+f 1 5 8
+f 1 8 4
 
-# Right face
-f 3 7 8
-f 3 8 4
+# Right face (normal +X, CCW from outside = looking from +X)
+f 2 3 7
+f 2 7 6
 
-# Bottom face
-f 1 4 8
-f 1 8 5
+# Bottom face (normal -Y, CCW from outside = looking from -Y)
+f 1 2 6
+f 1 6 5
 
-# Top face
-f 2 6 7
-f 2 7 3
+# Top face (normal +Y, CCW from outside = looking from +Y)
+f 4 8 7
+f 4 7 3