The four standalone installers pruned the oldest excess stamp
directories regardless of origin, so a migration retiring more than
five batches destroyed its own rollback material mid-run — the same
data loss already fixed for Go via the run-root registry and present
in install.js/install.ps1 as currentRunBackupRoots. Every installer now
records the stamp directories it creates and pruning only removes
earlier-run batches, which is what the changelog already promises.