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.

docs/systems/social.md

@@ -500,7 +500,7 @@ All handlers also update `discoverStore` relationship state via lazy import.
 
 | WS Event | Store Method | Effect |
 |----------|-------------|--------|
-| `presence_update` | `updateFriendPresence(userId, status)` | Updates `status` field on matching friend by ID (all origins) |
+| `presence_update` | `updateFriendPresence(userId, status)` | Updates `status` on matching friend by ID (all origins). Server broadcasts to friends + DM co-members + space co-members (`collectProfileBroadcastTargetIds`). For federated friends, status is projected by the home instance via S2S `presence_update` relay (see `federation.md` §10 — Presence Sync) and broadcast to the same recipient set on the receiving instance. |
 | `user_updated` | `updateFriendProfile(user)` | Updates displayName, avatar, banner, accentColor, avatarColor, bio, customStatus, status on matching friend by ID |
 
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