fix: push presence when a player comes back online

OnDisconnectedAsync told friends a player went offline, but nothing ever
pushed the opposite transition — PlayerCameOnline existed in
NotificationService with zero callers. The pair was asymmetric, so a SignalR
reconnect (network blip, replica restart) greyed the player out on every
friend's list for the rest of the session even though they were still
playing. Only re-entering a room, which pushes presence via /goto, brought
them back.

AddConnectionAsync already computed the first-connection transition to
maintain the online set but returned void; it now reports it, mirroring
RemoveConnectionAsync.

The push is deliberately conditional on a room already being known. The
presence payload derives isOnline from roomInstance being non-null, so
pushing with no room reads as OFFLINE — worse than staying quiet. On a fresh
boot there is no room yet and the /goto that follows sends the real presence
anyway, so this only fires on reconnects, where the cached room is exactly
what friends need to see again.

docs/recroom-2023-client-api-complete.md

@@ -235,6 +235,7 @@ verified against the binary before the change, and the full test suite passes.
 | `SignalR push `Id` (notify)` | enum NAME sent for most ids; client only registered numbers — pushes vanished | numeric by default, with the six string-keyed ids listed explicitly |
 | `SignalR SubscriptionUpdateRoom` | sent as `SubscriptionUpdateRoom`; client listens on `RoomUpdate` | sent as `RoomUpdate` |
 | `Room ban notification` | sent id 24, for which the client has no handler | sent as ModerationKick (22) with IsBan |
+| `Presence on SignalR reconnect` | disconnect pushed offline; nothing ever pushed back online, so a reconnect greyed the player out for the rest of the session | connect re-pushes presence when a room is already known |
 
 Everything else in the per-subsystem Defects sections below is still
 outstanding.