Fix Friend.Sync deadlock via consistent lock ordering

Friend.Sync() locked source then target. If two users interacted
simultaneously with swapped source/target, ABBA deadlock occurred.
Fix by always locking the object with the lower identity hash first.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: wjlafrance

src/Atlasd/Battlenet/Friend.cs

@@ -60,9 +60,20 @@ public void Sync(GameState source)
                 return;
             }
 
-            lock (source)
+            // Enforce consistent lock ordering to prevent ABBA deadlock.
+            // Always lock the object with the lower RuntimeHelpers hash first.
+            var first = source;
+            var second = target;
+            if (System.Runtime.CompilerServices.RuntimeHelpers.GetHashCode(source) >
+                System.Runtime.CompilerServices.RuntimeHelpers.GetHashCode(target))
             {
-                lock (target)
+                first = target;
+                second = source;
+            }
+
+            lock (first)
+            {
+                lock (second)
                 {
                     var admin = source.HasAdmin();
                     var mutual = false;