docs(federation): document username on profile_update + new presence_update relay + stub backfill
- federation.md §10: extend FederationProfileUpdatePayload with username, document
receiver fallback. Add Presence Sync sub-section: event shape, sender call sites,
outbox-only (no mutation log) policy, peer-lifecycle hooks, flap recovery
semantics. Add Stub Username Backfill sub-section + new
/api/federation/users/by-home-id endpoint.
- activity-presence.md: resolve drift — line 147 previously claimed S2S
presence_update relays existed but the code didn't ship them. Now points to
federation.md §10 which describes the actually-implemented mechanism. Connect/
Disconnect Flow updated to reflect collectProfileBroadcastTargetIds recipient
set + S2S queueing.
- social.md / websocket.md: presence_update recipient column now reflects
friends + DM + space co-members (matches user_updated), plus federated stub
presence sourced via S2S.
Jannis Braun
ba0f8637f5