0224 — Partition labs swap message_log to a lab-scale width and restore the schema on exit

Context. message_log_0’s real 1,000,000-row partition width makes multi-partition demos impractical at lab scale, but the partition-pruning and drop-floor labs need several partitions to prove anything.

Decision. partitionlab and dropfloorlab drop and recreate message_log at a lab-scale partition width for their own run, then restore the migration’s exact shape on exit. This permanently discards whatever rows were in message_log each time either lab runs — schema is restored, data is not — accepted as safe because no FK ties message_log to cursor/deliveries/lease. sweeplab needs no swap, since staying inside one never-rolled partition is exactly the condition the sweep exists to cover; it runs with AllowDropPastCommitted=true throughout because the drop floor is global across every group sharing message_log, so leftover cursor rows from other labs would otherwise couple its behavior to theirs.

Consequences. The labs prove automated pruning against the real datastore methods, at the cost of wiping message_log on every run. Rejected: a real migration-width config knob — deferred rather than added for a lab’s sake. Rejected: skipping the automated pruning proof outright. Superseded by the per-topic tables split (0241): a lab-scale partition width became just a PartitionSize passed to topic.Register, and the drop-and-recreate schema-swap was deleted entirely.