A host that handed the room to someone else and then lost its connection
could not get back in, and a guest promoted while it was offline came back
as a guest.
The relay names the host in every admission and every hostChanged, but the
client also kept its own _isHost flag beside that identity. Reconnect adopted
the relay's host id without recomputing the flag, so the two disagreed: a
demoted host still required the response to name itself and rejected its own
legitimate re-admission as an invalid response, retry after retry; a promoted
guest updated the session but left the transport a guest, so ending the
session sent leave instead of endSession and room re-creation stayed off.
The role is now derived from the host identity rather than stored next to it.
Before the relay has admitted us there is no authority yet, so the role is
the one we announced, which is what releasing a possibly-committed setup has
to go by. The identity assertion moves from mutable role state to message
semantics: a created response must name us, a joined response is adopted.
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