A promoted host started playing while other participants were still loading,
and could run at a different speed than the one it told the room.
The coordinator resolves the first epoch's readiness synchronously inside
attach, so seeding the known-peer roster afterwards had already missed it:
the fresh epoch saw an empty room, solo-started, and marked the first start
complete, after which the still-loading peers no longer gated anything. The
roster now seeds in _createCoordinator, so no coordinator exists without the
room it has to wait for.
Adopting the room's rate on promotion only set the value the coordinator
broadcasts. A guest that was paused when the rate changed never applied it to
its player -- position is aligned while stopped, rate is not -- so the new
host advertised one speed and ran another, and every guest kept correcting
against the difference. The host player is the room clock, so an adopted rate
is now applied to it.
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