Routes external signals and child-completion wakeups to workflow processes.

With the Postgres journal, signals are durable: delivery inserts a row into continuum_signals, emits pg_notify('continuum_signal', run_id), and wakes a local engine when one is registered. The engine consumes the mailbox row into a journaled signal_received event during replay.

This process also listens on continuum_run_wake — emitted when a child run reaches a terminal state — and wakes the parent's local engine so an awaiting parent resumes promptly. No separate listener process: a parent wakeup routes through the same "find local pid in Registry, wake it, else rely on the Dispatcher poll" path as a signal.