Per-face-tagged glTF (cost-driver heatmap model) for a Part.

Same dataset-resolution + {url, status} contract as /parts/{id}/preview-url but serves faces_glb_storage_key — the model where every B-rep face is its own primitive tagged with face_uuid. Only produced for STEP inputs; the frontend falls back to the opaque preview when url is None.

part_revision_id (optional query param) pins resolution to a specific revision so the heatmap's face_uuids line up with the cost drivers from a calc on THAT revision. Without it, the newest revision's dataset is served (a re-upload after the calc would otherwise blank the heatmap — the GLB's face_uuids would derive from different STEP bytes than the drivers).