Extract the conn-event handling into a testable ConnEventHandler (grace window
as a field) and add tests/reconnect_eviction.rs covering:
- second outage after a reconnect still evicts (the bug fixed in bc1a0a2;
proven to fail when the transport-arming is disabled)
- a reconnect within grace is not evicted
- a first-ever dial is not given an eviction timer
- a graceful leave scrubs seen_connected so a later rejoin dials cleanly
Behavior-preserving refactor: the conn-event task now builds a ConnEventHandler
and forwards each event to it; arm_grace_timer takes the grace Duration as a
param (production passes RECONNECT_GRACE). All existing transport/reconnect
integration tests still pass; clippy clean.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>