macos: isSplit guarding on focus split directions works

macos/Sources/Features/Splits/SplitTree.swift

@@ -72,7 +72,12 @@ extension SplitTree {
     var isEmpty: Bool {
         root == nil
     }
-    
+
+    /// Returns true if this tree is split.
+    var isSplit: Bool {
+        if case .split = root { true } else { false }
+    }
+
     init() {
         self.init(root: nil, zoomed: nil)
     }

macos/Sources/Ghostty/Ghostty.App.swift

@@ -921,8 +921,7 @@ extension Ghostty {
                     // we should only be returning true if we actually performed the action,
                     // but this handles the most common case of caring about goto_split performability
                     // which is the no-split case.
-                    // TODO: fix this
-                    //guard controller.surfaceTree?.isSplit ?? false else { return false }
+                    guard controller.surfaceTree2.isSplit else { return false }
 
                     NotificationCenter.default.post(
                         name: Notification.ghosttyFocusSplit,