Context.Watch each spawned child; OnChildTerminated evicts it from _children so the
next ApplyVirtualTags (still containing that vtagId) falls through the ContainsKey
guard and re-spawns a fresh VirtualTagActor. Adds a spawn-site Debug log, moves the
TODO about in-place plan mutation to the skip-existing branch where it belongs, and
adds a deterministic TestKit test (Child_is_respawned_after_unexpected_termination)
that kills the first child, drains its UnregisterInterest from the mux probe, re-applies,
and asserts a second distinct RegisterInterest arrives.
Joseph Doherty
5e2869bab7