feat(macos): foundation of the macOS app ported from Swift to Rust

Phase 9, MA1: crates/ghostty-macos (AppKit via objc2; empty stub on
non-Apple targets) ports the foundation of Ghostty 1.3.1's Swift macOS
app (ghostty/macos): main, AppDelegate (+Ghostty) with lifecycle, dock
menu, open-file, notifications, shortcut sync and quit/reopen paths;
MainMenu.xib as generated menu tables built into NSMenus; the
Ghostty.App/Config/Surface wrappers driving libghostty in-process via
ghostty-lib; the generic helpers; scripts/macos-bundle.sh to assemble
Ghostty.app (dry-run on Linux). Feature areas are stubbed for MA2-MA5.
Design and type mapping: docs/rewrite/notes/macos-app-design.md
(App Intents is a Swift-only framework and cannot be ported).

Compile-checked only (no macOS here): just check-macos passes for
aarch64/x86_64-apple-darwin with -D warnings; 125 platform-independent
tests run on Linux.

Test Plan:
- just check-macos
- cargo test -p ghostty-macos

crates/ghostty-macos/src/helpers/extensions/mod.rs

@@ -0,0 +1,68 @@
+//! `Helpers/Extensions/*`: one module per Swift file. A Swift `extension X`
+//! becomes an extension trait `XExt` implemented for the objc2 type (or free
+//! functions for value types). Pure logic is in the platform independent
+//! modules; the AppKit traits are `cfg(target_vendor = "apple")`.
+//!
+//! Unportable (SwiftUI only): `View+Extension.swift`, `Transferable+Extension.swift`.
+//! `UUID+Extension` (CFUUID) lives in [`uuid`]; `KeyboardShortcut+Extension`
+//! is `helpers::keyboard_shortcut`.
+
+// Platform independent.
+pub mod array;
+pub mod double;
+pub mod duration;
+pub mod event_modifiers;
+pub mod file_handle;
+pub mod ns_screen_logic;
+pub mod ns_window_logic;
+pub mod object_identifier;
+pub mod optional;
+pub mod os_color;
+pub mod string;
+
+// AppKit.
+#[cfg(target_vendor = "apple")]
+pub mod ns_appearance;
+#[cfg(target_vendor = "apple")]
+pub mod ns_application;
+#[cfg(target_vendor = "apple")]
+pub mod ns_color;
+#[cfg(target_vendor = "apple")]
+pub mod ns_image;
+#[cfg(target_vendor = "apple")]
+pub mod ns_menu;
+#[cfg(target_vendor = "apple")]
+pub mod ns_menu_item;
+#[cfg(target_vendor = "apple")]
+pub mod ns_notification;
+#[cfg(target_vendor = "apple")]
+pub mod ns_pasteboard;
+#[cfg(target_vendor = "apple")]
+pub mod ns_screen;
+#[cfg(target_vendor = "apple")]
+pub mod ns_view;
+#[cfg(target_vendor = "apple")]
+pub mod ns_window;
+#[cfg(target_vendor = "apple")]
+pub mod ns_workspace;
+#[cfg(target_vendor = "apple")]
+pub mod undo_manager;
+#[cfg(target_vendor = "apple")]
+pub mod user_defaults;
+#[cfg(target_vendor = "apple")]
+pub mod uuid;
+
+/// `#available(macOS N, *)`: true if the running OS is at least `major`.
+#[cfg(target_vendor = "apple")]
+#[must_use]
+pub fn macos_at_least(major: isize) -> bool {
+    use objc2_foundation::{NSOperatingSystemVersion, NSProcessInfo};
+    NSProcessInfo::processInfo().isOperatingSystemAtLeastVersion(NSOperatingSystemVersion {
+        majorVersion: major,
+        minorVersion: 0,
+        patchVersion: 0,
+    })
+}
+
+#[cfg(target_vendor = "apple")]
+pub mod uti;