Apply a partial diff to the vendor's import draft. Designed for debounced auto-save — send only the fields that changed.

Semantics:

• add_selection — append uniquely
• remove_selection — remove these IDs and delete their row entries
• rows_upsert — merge each row patch into the existing row (keys not in selection are ignored)
• rows_remove — delete row entries but keep the IDs in selection (clears fill data only)
• defaults — shallow-merge into existing defaults

If no draft exists, one is created.