Plain field-copy materialization so the persistence round-trip closes.
When OrderLines arrive, FromOrderData becomes the one-line reconstruction
(row description + total become a single line) — the hinge Exercise 2
pivots on.
yak: orderdata-read-back-identity-version-j9ew [order-data-mapping]
The fake DbContext now stores data-only DbBase subclasses (renamed from
the identity-only Shadow wrappers) and refreshes rows in place on re-Save;
the object graph is flattened to ids in storage.
yak: data-carrying-persistence-records-orderdataclientdata-mappers-tb3a