Returns %{source_uuid => %{item_uuid => Decimal}} — for each uuid in source_uuids, the quantity already committed against it, summed across all non-deleted rows of schema whose source_refs contain a ref whose "type" is in ref_types and whose "uuid" matches.

For refs recorded with a "lines" breakdown, uses that breakdown exactly. For legacy refs without one, falls back to attributing the document's own aggregate lines (keyed by line_quantity_field) to that ref — a safe overcount for the rare pre-existing multi-source merge.

Merges a {ref_type, source_uuid} ref carrying imported_lines (%{item_uuid => Decimal}) into existing_refs. If a ref for that {ref_type, source_uuid} pair already exists, its "lines" map is summed with imported_lines in place — never skipped, never replaced — so the ref's "lines" always reflects the running cumulative total ever pulled from that source into this document.