Optional material override for Command3D.DrawModelWith / Command3D.DrawAnimatedModelWith.

All applies one Material3D to every mesh part (allocation-free struct field). PerMesh takes a resolver indexed by a flat counter over model.Meshes × MeshParts in pipeline iteration order.

The PerMesh resolver is invoked at least once per mesh part per frame on the forward pass and again during shadow collection (plus additional calls under user-effect scopes), with no memoization. It must be pure and cheap — prefer returning a precomputed material over allocating or computing per call.