Get Deliverable Parts

Generic KG parts-read (JEEVY-649): the deliverable's per-line bom-line nodes + their intrinsic dimension/material facts (od/length/schedule/nominal_size/ material_type), folded from the KG observation ledger.

The read-side realization of "render from the KG" — any category viewer reads deliverable → lines → facts here (the pig viewer is the first consumer, reading the facts the pig enricher decorates onto the per-line bom-line nodes). GENERIC: no category vocabulary. Grain = bom-line (per-line, master ruling option 3): the dims are per-line facts, and the deduped part node collapses to part:- without an order_no, so the per-line node keeps distinct dims per item. The KG walk lives in the service layer (list_deliverable_parts). 404 when the deliverable does not exist.