feat: Extend Save3D to save vertex colors and textures (CORE-189) (#13824)

Split GLB save logic out of nodes_hunyuan3d.py into a new nodes_save_3d.py, and extend the writer to support UVs, per-vertex colors, and embedded baseColor textures.

Extend the MESH type with optional uvs, vertex_colors, and texture fields so meshes can carry texture data through the graph.

Add pack_variable_mesh_batch / get_mesh_batch_item helpers and switch VoxelToMesh / VoxelToMeshBasic to use them so batches with differing vertex/face counts no longer fail at torch.stack.

comfy_api/latest/_util/geometry_types.py

@@ -12,9 +12,24 @@ class VOXEL:
 
 
 class MESH:
-    def __init__(self, vertices: torch.Tensor, faces: torch.Tensor):
-        self.vertices = vertices
-        self.faces = faces
+    def __init__(self, vertices: torch.Tensor, faces: torch.Tensor,
+                 uvs: torch.Tensor | None = None,
+                 vertex_colors: torch.Tensor | None = None,
+                 texture: torch.Tensor | None = None,
+                 vertex_counts: torch.Tensor | None = None,
+                 face_counts: torch.Tensor | None = None):
+
+        assert (vertex_counts is None) == (face_counts is None), \
+            "vertex_counts and face_counts must be provided together (both or neither)"
+        self.vertices = vertices            # vertices: (B, N, 3)
+        self.faces = faces                  # faces: (B, M, 3)
+        self.uvs = uvs                      # uvs: (B, N, 2)
+        self.vertex_colors = vertex_colors  # vertex_colors: (B, N, 3 or 4)
+        self.texture = texture              # texture: (B, H, W, 3)
+        # When vertices/faces are zero-padded to a common N/M across the batch (variable-size mesh batch),
+        # these hold the real per-item lengths (B,). None means rows are uniform and no slicing is needed.
+        self.vertex_counts = vertex_counts
+        self.face_counts = face_counts
 
 
 class File3D: