The receiver, resubmit and replay handlers now read and check the
request and make one call. Engine.StoreEvent stores an event with a
pending delivery for each active target and the totals in one
transaction, then queues them; Engine.CreateReplay does the same for
one replay delivery. The handlers take *delivery.Engine, so the
Notifier interface and its bridge are gone, and the engine's notify is
no longer exported. Archives and CircuitBreakers stay because the
handler tests fake them. The tests that read a queued task moved to
the engine's tests; the handler tests use an engine that is never
started.
Model: opus-5-5
