macos: add terminals element to window and tab AppleScript classes

Expose terminal surfaces as elements on both ScriptWindow and ScriptTab,
allowing AppleScript to enumerate terminals scoped to a specific window
or tab (e.g. `terminals of window 1`, `terminals of tab 1 of window 1`).

Changes:
- Add `<element type="terminal">` to window and tab classes in Ghostty.sdef
- Add `terminals` computed property and `valueInTerminalsWithUniqueID:`
  lookup to ScriptWindow (returns all surfaces across all tabs)
- Add `terminals` computed property and `valueInTerminalsWithUniqueID:`
  lookup to ScriptTab (returns surfaces within that tab)

macos/Sources/Features/AppleScript/ScriptWindow.swift

@@ -73,6 +73,25 @@ final class ScriptWindow: NSObject {
         return ScriptTab(window: self, controller: controller)
     }
 
+    /// Exposed as the AppleScript `terminals` element on a window.
+    ///
+    /// Returns all terminal surfaces across every tab in this window.
+    @objc(terminals)
+    var terminals: [ScriptTerminal] {
+        controllers
+            .flatMap { $0.surfaceTree.root?.leaves() ?? [] }
+            .map(ScriptTerminal.init)
+    }
+
+    /// Enables unique-ID lookup for `terminals` references on a window.
+    @objc(valueInTerminalsWithUniqueID:)
+    func valueInTerminals(uniqueID: String) -> ScriptTerminal? {
+        controllers
+            .flatMap { $0.surfaceTree.root?.leaves() ?? [] }
+            .first(where: { $0.id.uuidString == uniqueID })
+            .map(ScriptTerminal.init)
+    }
+
     /// AppleScript tab indexes are 1-based, so we add one to Swift's 0-based
     /// array index.
     func tabIndex(for controller: BaseTerminalController) -> Int? {