Mints block and document ids: "blk_" <> uxid and "bdoc_" <> uxid, following the family's identifier convention (st-ADR-0008, as amended to drop the uxid dependency and mint inline - statifier-ex/lib/statifier/machine_state.ex:571-585).

Ids are opaque to this layer and to everything downstream of it (ADR-0001 decision 3): nothing here or elsewhere parses one back apart, and no code should come to depend on the prefix or body being anything other than an opaque, sortable-by-mint-time string.