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

Assets/_Project/Scripts/Client/UI/MainMenuController.cs

@@ -21,6 +21,7 @@ namespace ProjectM.Client
         VisualElement _howToPanel;
         TextField _ipField;
         Label _classLabel;
+        bool _autoStarted;
 
         void Awake()
         {
@@ -32,10 +33,12 @@ namespace ProjectM.Client
             // The menu owns the cursor.
             UnityEngine.Cursor.lockState = CursorLockMode.None;
             UnityEngine.Cursor.visible = true;
+            _autoStarted = TryAutoStartFromCommandLine();
         }
 
         void OnEnable()
         {
+            if (_autoStarted) return; // headless CLI co-op session; don't build the menu UI
             if (_doc == null) _doc = GetComponent<UIDocument>();
             var root = _doc.rootVisualElement;
             if (root == null) return;
@@ -43,6 +46,28 @@ namespace ProjectM.Client
             BuildMain(root);
         }
 
+        /// <summary>
+        /// Dev/automation hook: a player launched with <c>-mhost</c> auto-hosts; <c>-mjoin &lt;ip&gt;</c> auto-joins
+        /// (ip optional, defaults to loopback). Lets two standalone builds form a co-op session headlessly for
+        /// testing — the same <see cref="WorldLauncher.StartSession"/> path the menu buttons use, no UI clicks.
+        /// No effect when neither arg is present. Returns true if a session was started.
+        /// </summary>
+        static bool TryAutoStartFromCommandLine()
+        {
+            var args = System.Environment.GetCommandLineArgs();
+            for (int i = 0; i < args.Length; i++)
+            {
+                if (args[i] == "-mhost") { WorldLauncher.StartSession(SessionMode.Host, "", false); return true; }
+                if (args[i] == "-mjoin")
+                {
+                    string ip = (i + 1 < args.Length && !args[i + 1].StartsWith("-")) ? args[i + 1] : "127.0.0.1";
+                    WorldLauncher.StartSession(SessionMode.Join, ip, false);
+                    return true;
+                }
+            }
+            return false;
+        }
+
         static void EnsureMenuWorld()
         {
             var w = World.DefaultGameObjectInjectionWorld;