Schema migration half-applied across shards

A schema migration adds a currency column (defaulting writes to a per-row value) across a 16-shard MySQL fleet for an orders table. The migration runner applies DDL shard-by-shard. At 03:20 it failed partway: shards 0–9 have the new column, shards 10–15 don't. The runner reported failure and paged, but the API kept serving. Now reads are inconsistent: orders on migrated shards return a currency, orders on un-migrated shards 500 on the new code path that expects the column, and a reporting job is summing amounts across currencies as if they were all USD. How do you triage, stabilize, and safely complete the migration?