Bound admission and isolate multiplexed contexts

For a realtime proxy, enforce immediate bounded admission at global, per-connection, and per-routing-key scopes, with independent provider caps and circuit breakers. Bound outbound queues and emit explicit rejection metrics instead of allowing unbounded queues.

When several logical contexts share one upstream connection, do not await untimed per-context delivery serially: one slow context can head-of-line block siblings. Transfer this pattern between speech services only when event delivery remains isolated, bounded, and timed.