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/main.rs

@@ -0,0 +1,13 @@
+//! `ghostty-macos-app`: the executable inside `Ghostty.app` (Swift `main.swift`).
+//! An empty stub on non-Apple targets.
+
+#[cfg(target_vendor = "apple")]
+fn main() {
+    ghostty_macos::app::entry::run();
+}
+
+#[cfg(not(target_vendor = "apple"))]
+fn main() {
+    eprintln!("ghostty-macos-app only runs on macOS");
+    std::process::exit(1);
+}