hostRoomKnowsItsRoomIdAtConstruction

A host knows which room it is at construction — Room.roomId is non-null the moment RoomFactory.host returns, with no round trip and nothing to wait for.

And it names the room, not the host: a second room from the same factory (hence the same selfId) gets a different id. That is the whole of #1594 — a fabric whose selfId happens to be fresh per weave would make the first assertion pass and this one vacuous, so both are here.