050-split-cell-coherent-export-and-archive-identity

Split cell coherent export and archive identity: an offline export across both clusters, and one archive manifest naming both images and both applied log ids

Takes over spec 044's B-11 (coherent offline export across the rahi and Rauthy clusters of a split N=3 cell) and B-12 (the cell archive names both images, both applied log ids and both digests in one manifest), which the owner split out on 2026-10-31 so 044 could complete on its operator evidence. Both need hiqlite fork changes that no release carries yet: the offline export entry point, and an applied log id recorded with each backup. This spec schedules nothing until a hiqlite release carrying both is adopted.