Steps: (1) OrderLine + computed total — the compiler enumerates the writers,
(2) one-line reconstruction in the two inbound mappers, (3) discount on a
reloaded order + the accepted-loss test. Dry-run verified on a scratch
branch: 98 tests green, diff confined to domain/builder/two mappers.
yak: exercise2md-orderlines-evolution-sequenced-accepted-loss-test-efap
