Supply Shipments: Track Lot Numbers On Items

Supply shipment items now carry a lot number, so one PO line can be split into multiple shipment rows — same SKU, different production batches — each with its own quantity and its own lot. Assign lot numbers when adding items to a shipment, edit or split an existing line across lots, and filter shipments by lot number from the list. Lot-aware product costs surface on rows where they're relevant. Coverage math sums per lot bucket, closing a case where two lots on the same PO line collided and one row's quantity silently vanished from the totals.

WMS/ASN payloads and lot-aware inbound sync aren't wired yet — those follow once the WMS side sends a lot identifier.