macOS: equalize splits when double tapping on SplitView divider (#9524)

Resolves Issue #8357 

### Implementation

Following the existing `onResize` callback pattern in
`TerminalSplitTreeView`, I added an `onEqualize` callback to
`SplitView`. When a divider is double-tapped, the callback retrieves a
surface from that `TerminalView`'s `SplitTree` and calls `splitEqualize`
to equalize the entire tree.

### Context

There is an existing PR #8364 that implements this feature but uses
`focusedSurface`, which doesn't work for unfocused windows. Since that
PR has been inactive for a few months after requested changes, I've
implemented this alternative approach.

Credit to @liby for that initial implementation!

### AI Usage

I chatted with Claude Code in Plan mode to understand the relationship
between surfaces and the split tree/split views, but I wrote all the
code myself.

### Screenshot


https://github.com/user-attachments/assets/0efd70ef-c90e-4b50-b853-b05e2ca2be67

macos/Sources/Features/Splits/SplitView.swift

@@ -21,6 +21,9 @@ struct SplitView<L: View, R: View>: View {
     let left: L
     let right: R
 
+    /// Called when the divider is double-tapped to equalize splits.
+    let onEqualize: () -> Void
+
     /// The minimum size (in points) of a split
     let minSize: CGFloat = 10
 
@@ -56,6 +59,9 @@ struct SplitView<L: View, R: View>: View {
                         split: $split)
                     .position(splitterPoint)
                     .gesture(dragGesture(geo.size, splitterPoint: splitterPoint))
+                    .onTapGesture(count: 2) {
+                        onEqualize()
+                    }
             }
             .accessibilityElement(children: .contain)
             .accessibilityLabel(splitViewLabel)
@@ -69,7 +75,8 @@ struct SplitView<L: View, R: View>: View {
         dividerColor: Color,
         resizeIncrements: NSSize = .init(width: 1, height: 1),
         @ViewBuilder left: (() -> L),
-        @ViewBuilder right: (() -> R)
+        @ViewBuilder right: (() -> R),
+        onEqualize: @escaping () -> Void
     ) {
         self.direction = direction
         self._split = split
@@ -77,6 +84,7 @@ struct SplitView<L: View, R: View>: View {
         self.resizeIncrements = resizeIncrements
         self.left = left()
         self.right = right()
+        self.onEqualize = onEqualize
     }
 
     private func dragGesture(_ size: CGSize, splitterPoint: CGPoint) -> some Gesture {

macos/Sources/Features/Splits/TerminalSplitTreeView.swift

@@ -55,6 +55,10 @@ struct TerminalSplitSubtreeView: View {
                 },
                 right: {
                     TerminalSplitSubtreeView(node: split.right, onResize: onResize)
+                },
+                onEqualize: {
+                    guard let surface = node.leftmostLeaf().surface else { return }
+                    ghostty.splitEqualize(surface: surface)
                 }
             )
         }

macos/Sources/Ghostty/InspectorView.swift

@@ -32,6 +32,9 @@ extension Ghostty {
                         InspectorViewRepresentable(surfaceView: surfaceView)
                             .focused($inspectorFocus)
                             .focusedValue(\.ghosttySurfaceView, surfaceView)
+                    }, onEqualize: {
+                        guard let surface = surfaceView.surface else { return }
+                        ghostty.splitEqualize(surface: surface)
                     })
                 }
             }