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/uuid.rs

@@ -0,0 +1,12 @@
+//! `Helpers/Extensions/UUID+Extension.swift`.
+
+use objc2::{AnyThread, rc::Retained};
+use objc2_core_foundation::CFUUID;
+use objc2_foundation::{NSString, NSUUID};
+
+/// `UUID(_ cfuuid: CFUUID)`.
+#[must_use]
+pub fn uuid_from_cfuuid(cfuuid: &CFUUID) -> Option<Retained<NSUUID>> {
+    let s = CFUUID::new_string(None, Some(cfuuid))?;
+    NSUUID::initWithUUIDString(NSUUID::alloc(), &NSString::from_str(&s.to_string()))
+}