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/mod.rs

@@ -0,0 +1,58 @@
+//! `Sources/Helpers`: the generic helpers.
+//!
+//! SwiftUI-only helpers have no Rust/AppKit counterpart and are documented as
+//! unportable in `docs/rewrite/notes/macos-app-design.md` (`Backport`,
+//! `HostingWindow`, `NonDraggableHostingView`, `MetalView`, the `View` /
+//! `Transferable` extensions). `Weak<T>` is `std::rc::Weak` / `objc2::rc::Weak`.
+//!
+//! Pure logic (string/color/frame math, counters, decisions) lives in
+//! platform independent modules so it is unit-tested on every target; the
+//! AppKit parts are thin `cfg(target_vendor = "apple")` wrappers around it.
+
+#![cfg_attr(test, allow(clippy::unwrap_used))]
+
+// Platform independent (tested on every target).
+pub mod any_sort_key;
+pub mod app_info;
+pub mod color;
+pub mod cursor_counter;
+pub mod extensions;
+pub mod fullscreen_logic;
+pub mod geometry;
+pub mod keyboard_shortcut;
+pub mod last_window_position_logic;
+pub mod notification_center;
+pub mod published;
+
+// AppKit.
+#[cfg(target_vendor = "apple")]
+pub mod codable_bridge;
+#[cfg(target_vendor = "apple")]
+pub mod cross_kit;
+#[cfg(target_vendor = "apple")]
+pub mod cursor;
+#[cfg(target_vendor = "apple")]
+pub mod dispatch;
+#[cfg(target_vendor = "apple")]
+pub mod expiring_undo_manager;
+#[cfg(target_vendor = "apple")]
+pub mod fullscreen;
+#[cfg(target_vendor = "apple")]
+pub mod keyboard_layout;
+#[cfg(target_vendor = "apple")]
+pub mod last_window_position;
+#[cfg(target_vendor = "apple")]
+pub mod objc_exception_catcher;
+#[cfg(target_vendor = "apple")]
+pub mod private;
+#[cfg(target_vendor = "apple")]
+pub mod vibrant_layer;
+
+/// The main thread marker. Every AppKit object is created and used on the main
+/// thread (the Swift app uses `@MainActor` / `DispatchQueue.main`); panics
+/// elsewhere, which is a programming error.
+#[cfg(target_vendor = "apple")]
+#[must_use]
+pub fn main_thread() -> objc2::MainThreadMarker {
+    objc2::MainThreadMarker::new().expect("must be called on the main thread")
+}