close#2183
Reusing a room means whoever enters first becomes the host, and moving the
session to someone else's library meant everyone leaving and re-entering in
the right order. The host can now promote any connected guest from the
participant list (Watch Together screen and the in-player session sheet),
with playback, control mode, and reconnect identities carried across.
The relay owns the swap: a new `transferHost` message validates the sender
is the live host and the target a connected modern-protocol guest, then
swaps `HostPeerID` and the reconnect verifiers (each peer keeps its own
token), persists the room, and broadcasts `hostChanged` to every peer.
Clients flip roles in place: the controller swaps its role engine while
keeping the session, message queue, and player attachment. A promoted guest
seeds the coordinator with the room's last known intent (a paused room
stays paused) and the known-peer roster so the fresh epoch re-gates
instead of solo-starting; a demoted host falls back to a reconciler with a
fresh clock sync and asks the new authority for state. Guests re-pin the
host identity, reset their sequence, and re-converge their clocks.
A rejected transfer (`not_host` / `peer_not_found`) surfaces as a toast
instead of tearing the session down. Pre-transfer app builds in the room
ignore `hostChanged` and stop following the room at the next transfer;
targets on an old sync protocol are not offered the action.
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