0226 — Retention shipped scoped to the one shared log, per-topic scoping deferred

Context. Retention was built on a single shared message_log: one RetentionTTL, and a drop floor of MIN(committed) across every cursor row regardless of which routing_key each group actually consumes.

Decision. Ship retention per-log anyway, with the known limitation filed rather than fixed: one lagging group on an unrelated stream blocks partition drops for everyone sharing the table. Kafka avoids this because retention.ms is per-topic and each topic is its own log; the filed note said the system may need an actual topic concept — its own log and partition set — instead of routing_key filtering over one shared table.

Consequences. Retention and the floor were system-wide until the topic concept landed. Superseded by per-topic tables (0241): each topic became its own message_log_<topic_id> and cursorFloor gained WHERE topic_id = $1, bounding a lagging group’s effect to its own topic.