Record the real PBR attribute layout (6 channels)

base_color 0:3, metallic 3:4, roughness 4:5, alpha 5:6 - the pipeline's own
pbr_attr_layout. o_voxel's baker indexes this dict BY NAME and raises KeyError deep
inside to_glb on any missing slot rather than at the call, so a partial layout looks
like a baker bug. Needed by the texture stage; recorded now while it is in hand.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

pixal3d_mlx/mesh.py

@@ -28,6 +28,16 @@ import numpy as np
 # Upstream fixes both: the model always works in a unit cube centred on the origin.
 AABB = [[-0.5, -0.5, -0.5], [0.5, 0.5, 0.5]]
 
+# How the texture decoder's 6 channels map to PBR slots (pipeline's pbr_attr_layout).
+# o_voxel indexes this dict by name and raises KeyError on any missing slot, so a
+# partial layout fails deep inside the baker rather than at the call.
+PBR_ATTR_LAYOUT = {
+    "base_color": slice(0, 3),
+    "metallic": slice(3, 4),
+    "roughness": slice(4, 5),
+    "alpha": slice(5, 6),
+}
+
 
 def _torch(a):
     import torch