processPresenceUpdateEvent updates the local stub's status and broadcasts a
WS presence_update to friends + DM members + space co-members of that stub
via collectProfileBroadcastTargetIds. Closes the doc/code drift in
activity-presence.md:147 — federated stubs now have their status projected
by the home instance as documented.
Strict attribution: payload.homeInstance domain must equal source peer
domain. Silently no-ops when no local replica exists (peer broadcast fanout
covers all peers, not all hold a stub).
Jannis Braun