Load a vendor export whose schema changed, without losing rows

A CSV to warehouse loader falls over on the vendor's May export. Upstream renamed region_code to market, split customer_name into two columns, added an optional discount_cents, and switched the placed date from YYYY-MM-DD to DD/MM/YYYY, all without telling anyone. The loader raises on the first row. The tempting repairs are worse than the crash: a blanket except around the row loop silently drops rows, and mapping the new columns straight through changes the output field set that the warehouse and everything downstream of it are built on. The contract is written down in CONTRACT.md, including four reject reasons of which the loader has only ever implemented one, and April's export has to keep loading because the warehouse reloads history.