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.
