Every DM arriving in a ready payload with a federatedId now gets its
(origin, localChannelId) pair recorded in dmAlternatives, regardless of
whether the dedup pass kept this copy in dmChannels. Enables client-side
DM origin failover: when the primary origin drops, we can look up an
alternate origin's local channel ID for the same federated DM.
Prep for #10 (DM origin failover on disconnect).