Why: with _txlock=immediate the whole migration pass holds the write lock; four racing migrators on a slow CI disk exceeded the 5s busy timeout (job 2285: SQLITE_BUSY in TestMigrateRacingSQLite) although the local tier passes.
Evidence: go test -count=3 on the migration atomicity suite green locally; CI failure log cited in the Dev PR.
Impact: local SQLite tier only - lock contention waits up to 30s instead of erroring; no schema or API change.
Refs: Dev PR #325; plan KTD8.