0248 — Two routing_key slices sharing one topic share that topic's drop floor, by design

Context. Splitting message_log into per-topic tables re-scoped the retention drop floor from system-wide to per-topic. Within a topic, the floor is still one MIN(committed) across that topic’s cursors, regardless of which routing_key slice each group reads.

Decision. Left unfixed deliberately: a lagging group reading only orders.us.* still blocks a partition drop that orders.eu.* — same topic, different slice — would otherwise be free to have. Retention and partitioning are topic-scoped, not per-consumer or per-routing_key-slice.

Consequences. The cross-contamination is contained, not eliminated. If two slices in one topic diverge badly enough in consumer lag for this to matter, the deliberate operational escape hatch is splitting them into separate topics — which the per-topic split enables manually rather than automates. This exact behavior is demonstrated in topiclab.