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/websocket.md

@@ -12,7 +12,7 @@ Source: `packages/server/src/ws/handler.ts`, `packages/server/src/ws/events.ts`
 2. Client sends `{ type: 'auth', token: '<jwt>' }` within 10 seconds
 3. Server validates token (rejects deleted users, tokens issued before `passwordChangedAt`)
 4. Server responds with `ready` event containing full client state
-5. Server updates user status to `online`, broadcasts `presence_update` to all user's spaces
+5. Server updates user status to `online`, broadcasts `presence_update` to friends + DM co-members + space co-members (via `collectProfileBroadcastTargetIds`); for native users, also queues a S2S `presence_update` relay to all active peers
 6. Heartbeat: server pings every 30s (RFC 6455 ping frames), dead connections detected after ~65s
 
 ---
@@ -123,7 +123,7 @@ Source: `packages/server/src/ws/handler.ts`, `packages/server/src/ws/events.ts`
 ### Presence & Activity
 | type | fields | scope |
 |------|--------|-------|
-| `presence_update` | userId, status, activities? | space (all members) |
+| `presence_update` | userId, status, activities? | friends + DM co-members + space co-members of the user (via `collectProfileBroadcastTargetIds`), plus self for multi-tab sync. For federated stubs, the local instance receives status via S2S `presence_update` relay from the home (see `federation.md` §10 — Presence Sync) and re-broadcasts to the same recipient set. |
 | `user_updated` | user | user |
 
 ### Space / Channel Management