macos: add AppleScript split command

Add a new `split` command to the AppleScript scripting dictionary that
splits a terminal in a given direction (right, left, down, up) and
returns the newly created terminal.

The command is exposed as:
  split terminal <terminal> direction <direction>

Also adds a `fourCharCode` String extension for converting four-character
ASCII strings to their FourCharCode (UInt32) representation.

macos/Sources/Features/AppleScript/ScriptSplitCommand.swift

@@ -0,0 +1,74 @@
+import AppKit
+
+/// Handler for the `split` AppleScript command defined in `Ghostty.sdef`.
+///
+/// Cocoa scripting instantiates this class because the command's `<cocoa>` element
+/// specifies `class="GhosttyScriptSplitCommand"`. The runtime calls
+/// `performDefaultImplementation()` to execute the command.
+@MainActor
+@objc(GhosttyScriptSplitCommand)
+final class ScriptSplitCommand: NSScriptCommand {
+    override func performDefaultImplementation() -> Any? {
+        guard let terminal = evaluatedArguments?["terminal"] as? ScriptTerminal else {
+            scriptErrorNumber = errAEParamMissed
+            scriptErrorString = "Missing terminal target."
+            return nil
+        }
+
+        guard let surfaceView = terminal.surfaceView else {
+            scriptErrorNumber = errAEEventFailed
+            scriptErrorString = "Terminal surface is no longer available."
+            return nil
+        }
+
+        guard let directionCode = evaluatedArguments?["direction"] as? UInt32,
+              let direction = ScriptSplitDirection(code: directionCode) else {
+            scriptErrorNumber = errAEParamMissed
+            scriptErrorString = "Missing or unknown split direction."
+            return nil
+        }
+
+        // Find the window controller that owns this surface.
+        guard let controller = surfaceView.window?.windowController as? BaseTerminalController else {
+            scriptErrorNumber = errAEEventFailed
+            scriptErrorString = "Terminal is not in a splittable window."
+            return nil
+        }
+
+        guard let newView = controller.newSplit(at: surfaceView, direction: direction.splitDirection) else {
+            scriptErrorNumber = errAEEventFailed
+            scriptErrorString = "Failed to create split."
+            return nil
+        }
+
+        return ScriptTerminal(surfaceView: newView)
+    }
+}
+
+/// Four-character codes matching the `split direction` enumeration in `Ghostty.sdef`.
+private enum ScriptSplitDirection {
+    case right
+    case left
+    case down
+    case up
+
+    init?(code: UInt32) {
+        switch code {
+        case "GSrt".fourCharCode: self = .right
+        case "GSlf".fourCharCode: self = .left
+        case "GSdn".fourCharCode: self = .down
+        case "GSup".fourCharCode: self = .up
+        default: return nil
+        }
+    }
+
+    var splitDirection: SplitTree<Ghostty.SurfaceView>.NewDirection {
+        switch self {
+        case .right: .right
+        case .left: .left
+        case .down: .down
+        case .up: .up
+        }
+    }
+}
+