Resolves any raw DM channel ID (primary or alternate-origin local ID)
to its primary dmChannels entry via dmAlternatives federatedId lookup.
Returns null for unknown IDs. Used by the dm_message_created handler
in a later commit to prevent phantom sidebar entries from alternate-
origin deliveries (closes a pre-existing group-DM bug and supports
post-failover routing).
Drops the given origin from every inner (origin→localId) map; removes
the outer federatedId entry when its inner map becomes empty. Keeps the
store from accumulating stale origin references across long sessions
with connect/disconnect churn.
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).
Jannis Braun