start
HOST-ONLY. Close the lobby and begin the session: run the freeze/ack round, then adopt the seam as us.tractat.kuilt.session.SessionRole.Host. Retries internally while membership churns; returns the admitted Room once every member has acknowledged.
Throws
if this peer is not currently host.
if the underlying seam tears mid-election (co-electors permanently gone). Terminal and retryable — never suspends indefinitely on a mid-2PC collapse.