a Migration::Chunked pass commits in bounded-memory chunks with resume cursors in the counts keyspace, so a crash mid-migration restarts from the last committed key instead of the beginning. per-migration applied markers (mig_applied|{name}) replace the global version as proof of having run: reduced-feature builds advanced the old global version even when they skipped feature-gated work, so legacy versions now adopt markers via cheap migration-specific proofs (hydrant-iv6) instead of blindly re-running or skipping. issue: hydrant-90c, hydrant-iv6