resolve_upgrade_base now reads the head's published version (abra.head_compose_version,
the coop-cloud.<stack>.version label) and, when the last-green warm-canonical version
equals it, steps back to the newest published version strictly older than head instead
of deploying a same-version no-op. warm_reconcile gains version_key + newest_older_version
(single coop-cloud ordering source; sort_versions refactored onto version_key, no behavior
change). Skip only when no older published predecessor exists. Step-back returns kind=version
so it inherits F1d-2 pinned-tag checkout. Extends tests/unit/test_upgrade_base.py (13 pass).
docker stack deploy doesn't prune services the head compose dropped (discourse PR#4 drops sidekiq),
leaving them orphaned on the base image. perform_upgrade now reconciles the live stack to the head
compose service set (lifecycle.prune_orphan_services). Makes the deployed stack faithfully reflect
the head — no test weakened. No-op when service sets match / compose unresolvable.
