DriverHostActor now tells the mux SourceRemoved(driverInstanceId)
wherever a driver object ends: StopChild (deploy removal and the stop
half of a config-change respawn) and an operator Restart. It is not
sent on a supervision restart, which re-uses the same driver. Before
this nothing sent it, so a removed driver's last Good stayed in the
mux and replayed to the next replay-opting subscriber as current.
DriverHostActorSourceRemovedTests drives a real mux with two real
driver children: after a deploy drops DI-1 only DI-2's value replays
(positive control: both replay before the removal), and a supervision
restart of DI-1 leaves its value replayable.
Residual (documented on EvictRetainedValues): a burst the old child
enqueued just before it stopped can still be forwarded after the
eviction; the next publish, a comms-loss fan or the grace sweep
bounds it.
Refs #731#734
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
Joseph Doherty
5de1d661bb