macOS: split tree zoom state should encode as path, not full node (#8505)

Fixes #8356

Zoom state should encode as a path so that it can be mapped to a
reference to the node in `root`. Previously, we were encoding a full
node which was instantiating an extra terminal on restore.

AI disclaimer: Claude code did the Codable implementation.

macos/Sources/Features/Splits/SplitTree.swift

@@ -1,7 +1,7 @@
 import AppKit
 
 /// SplitTree represents a tree of views that can be divided.
-struct SplitTree<ViewType: NSView & Codable>: Codable {
+struct SplitTree<ViewType: NSView & Codable> {
     /// The root of the tree. This can be nil to indicate the tree is empty.
     let root: Node?
 
@@ -29,12 +29,12 @@ struct SplitTree<ViewType: NSView & Codable>: Codable {
     }
 
     /// The path to a specific node in the tree.
-    struct Path {
+    struct Path: Codable {
         let path: [Component]
 
         var isEmpty: Bool { path.isEmpty }
 
-        enum Component {
+        enum Component: Codable {
             case left
             case right
         }
@@ -334,6 +334,60 @@ extension SplitTree {
     }
 }
 
+// MARK: SplitTree Codable
+
+fileprivate enum CodingKeys: String, CodingKey {
+    case version
+    case root
+    case zoomed
+
+    static let currentVersion: Int = 1
+}
+
+extension SplitTree: Codable {
+    init(from decoder: Decoder) throws {
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+
+        // Check version
+        let version = try container.decode(Int.self, forKey: .version)
+        guard version == CodingKeys.currentVersion else {
+            throw DecodingError.dataCorrupted(
+                DecodingError.Context(
+                    codingPath: decoder.codingPath,
+                    debugDescription: "Unsupported SplitTree version: \(version)"
+                )
+            )
+        }
+
+        // Decode root
+        self.root = try container.decodeIfPresent(Node.self, forKey: .root)
+
+        // Zoomed is encoded as its path. Get the path and then find it.
+        if let zoomedPath = try container.decodeIfPresent(Path.self, forKey: .zoomed),
+           let root = self.root {
+            self.zoomed = root.node(at: zoomedPath)
+        } else {
+            self.zoomed = nil
+        }
+    }
+
+    func encode(to encoder: Encoder) throws {
+        var container = encoder.container(keyedBy: CodingKeys.self)
+
+        // Encode version
+        try container.encode(CodingKeys.currentVersion, forKey: .version)
+
+        // Encode root
+        try container.encodeIfPresent(root, forKey: .root)
+
+        // Zoomed is encoded as its path since its a reference type. This lets us
+        // map it on decode back to the correct node in root.
+        if let zoomed, let path = root?.path(to: zoomed) {
+            try container.encode(path, forKey: .zoomed)
+        }
+    }
+}
+
 // MARK: SplitTree.Node
 
 extension SplitTree.Node {

macos/Sources/Features/Terminal/TerminalRestorable.swift

@@ -4,7 +4,7 @@ import Cocoa
 class TerminalRestorableState: Codable {
     static let selfKey = "state"
     static let versionKey = "version"
-    static let version: Int = 4
+    static let version: Int = 5
 
     let focusedSurface: String?
     let surfaceTree: SplitTree<Ghostty.SurfaceView>