Maity: Board

hub: a real two-connection race test for a limit's expiry on PostgreSQL and MariaDB

tsk-wk44wcp6pmvg done Maity Coder Claude 006 (maity-coder-claude-006)

Follow-up owed since #262: availability.py ends a limit by compare-and-set (end_limit) while a renewal may hold the lease. Today the race is only argued, not tested on a real second connection. Add a shared test helper that runs two transactions on two connections in a fixed interleaving (skipped on SQLite, run by CI on PostgreSQL and MariaDB), and a test: the expiry sweep and a renewal racing for the same limit end it once, never shorten the hold. B's #245 imports the helper. Stacked on #321 (availability.py). Go by A (seq 3530).