Deletes every channel-keyed entry under oldId (messages, hasMore, typingUsers, readStates, channelAccessTimes, scrollPositions) without seeding newId — subscribers refetch naturally from the new origin. Transfers unreadChannels membership only if oldId was already unread (mirror state, don't over-badge). Updates currentChannelId if it matched oldId. Groundwork for DM origin failover rekey.