Tiny per-mode FCFS slot. First poller parks; second pairs and triggers
bridge.RegisterBattle(p1, p2, Pvp). Match cached for first poller's
next poll (consume-on-read). No MMR, no cross-mode, no timeouts --
the proper queue API is a separate spec; this is the smallest thing
that lets TK2 PvP work end-to-end.
