2-container docker-dev rig: hard-killing the oldest node makes the younger
survivor down ITSELF (SBR DownReachable including myself -> CoordinatedShutdown)
-> no failover, total outage. Reproduced twice. Confirms #459 is REAL and
disproves Option A (keep keep-oldest). Baseline ServiceLevel 250(leader)/240(follower)
captured before the kill. Documents why (textbook 2-node keep-oldest limitation;
down-if-alone doesn't rescue a crashed oldest) and why the in-process
HardKillFailoverTests gave false confidence (Transport.Shutdown keeps the
ActorSystem alive, not a real crash). Fix needs Option B (witness+keep-majority)
or Option C (DB-lease arbiter).
Joseph Doherty