feat(battle-engine): AOT/SFB/Steam/DisallowMultiple + FriendDataBase.SetPlayerData

Clears the last type+header frontier (RoomInviteFriendColum override). Per F3 this
unmasks the remaining View/UI/god-object member bodies (~8k) -- the next grind is
pure member-surface growth, closure (~3242 files) now essentially complete.

SVSim.BattleEngine/Shim/External/LooseEnds.cs

@@ -49,8 +49,20 @@ namespace Steamworks
     public sealed class Callback<T> { }
     // Microtransaction auth response struct (referenced as a callback type arg).
     public struct MicroTxnAuthorizationResponse_t { }
+    // Steam warning-message hook delegate (referenced as a type only).
+    public delegate void SteamAPIWarningMessageHook_t(int severity, System.Text.StringBuilder debugText);
 }
 
+// AOT P/Invoke callback attribute (IL2CPP) + StandaloneFileBrowser anchor.
+namespace AOT
+{
+    public sealed class MonoPInvokeCallbackAttribute : System.Attribute
+    {
+        public MonoPInvokeCallbackAttribute(System.Type type) { }
+    }
+}
+namespace SFB { internal class _ShimAnchor { } }
+
 // ---- third-party SDK namespace anchors (referenced via `using`) ----
 namespace Facebook { internal class _ShimAnchor { } }
 namespace Facebook.Unity { internal class _ShimAnchor { } }