Fix co-op join disconnect: gate client go-in-game on subscene load

A remote/cold client added NetworkStreamInGame the instant it got a
NetworkId, before the gameplay subscene's ghost prefabs finished
streaming. The server's first ghost snapshot then hit a client that
couldn't resolve those prefabs -> "ghost ... ENTITY_NOT_FOUND" ->
forced disconnect ("nothing loads"). On loopback / fast LAN the
go-in-game handshake beats the ~0.4s entity-subscene stream. The host
never hit it (it shares the server's loaded subscene); every remote
join did -- MPPM virtual players and real networked clients alike.

GoInGameClientSystem now gates full-client go-in-game on the gameplay
subscene being loaded (HasSingleton<PlayerSpawner>); thin clients skip
the gate. Verified cross-process in a standalone build over loopback:
join ghost-prefab errors 15->0, disconnects 1->0, client stays
connected and resolves all server ghosts.

Also adds a -mhost / -mjoin <ip> command-line hook to MainMenuController
for headless co-op testing (drives WorldLauncher.StartSession, no UI).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/_Project/Scripts/Client/Connection/GoInGameClientSystem.cs

@@ -29,6 +29,17 @@ namespace ProjectM.Client
         [BurstCompile]
         public void OnUpdate(ref SystemState state)
         {
+            // A FULL client must not go in-game until the gameplay subscene's ghost prefabs have streamed in.
+            // Otherwise the server's first ghost snapshot arrives before the client can resolve those prefabs
+            // ("ghost ... ENTITY_NOT_FOUND" -> the server disconnects the connection -> "nothing loads"). On
+            // loopback / fast LAN the connect+go-in-game handshake easily beats the ~0.5s entity-subscene stream.
+            // PlayerSpawner is a subscene-baked singleton that co-loads with the ghost prefabs, so its presence
+            // is a sound "subscene ready" gate. Thin clients never instantiate ghosts (and don't stream the
+            // subscene), so they skip the gate and connect immediately.
+            bool isThinClient = (state.WorldUnmanaged.Flags & WorldFlags.GameThinClient) == WorldFlags.GameThinClient;
+            if (!isThinClient && !SystemAPI.HasSingleton<PlayerSpawner>())
+                return;
+
             var ecb = new EntityCommandBuffer(Allocator.Temp);
 
             foreach (var (_, connection) in