macos: fix small memory leak in surface tree when closing splits

This fixes a small memory leak I found where the `SplitNode.Leaf` was
not being deinitialized properly when closing a split. It would get
deinitialized the next time a split was made or the window was closed,
so the leak wasn't big. The surface view underneath the split was also
properly deinitialized because we forced it, so again, the leak was
quite small.

But conceptually this is a big problem, because when we change the
surface tree we expect the deinit chain to propagate properly through
the whole thing, _including_ to the SurfaceView.

This fixes that by removing the `id(node)` call. I don't find this to be
necessary anymore. I don't know when that happened but we've changed
quite a lot in our split system since it was introduced. I'm also not
100% sure why the `id(node)` was causing a strong reference to begin
with... which bothers me a bit.

AI note: While I manually hunted this down, I started up Claude Code and
Codex in separate tabs to also hunt for the memory leak. They both
failed to find it and offered solutions that didn't work.

macos/Sources/Ghostty/Ghostty.TerminalSplit.swift

@@ -75,7 +75,6 @@ extension Ghostty {
                         .onReceive(pubZoom) { onZoom(notification: $0) }
                     }
                 }
-                .id(node) // Needed for change detection on node
             } else {
                 // On these events we want to reset the split state and call it.
                 let pubSplit = center.publisher(for: Notification.ghosttyNewSplit, object: zoomedSurface!)
@@ -289,7 +288,7 @@ extension Ghostty {
 
         let neighbors: SplitNode.Neighbors
         @Binding var node: SplitNode?
-        @StateObject var container: SplitNode.Container
+        @ObservedObject var container: SplitNode.Container
 
         var body: some View {
             SplitView(
@@ -331,7 +330,6 @@ extension Ghostty {
                 }
 
                 // Closing
-                container.topLeft.close()
                 node = container.bottomRight
 
                 switch (node) {
@@ -362,7 +360,6 @@ extension Ghostty {
                 }
 
                 // Closing
-                container.bottomRight.close()
                 node = container.topLeft
 
                 switch (node) {