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/lib.rs

@@ -0,0 +1,28 @@
+//! Ghostty's macOS app: a Rust/AppKit port of the Swift app in `macos/Sources`.
+//!
+//! The Swift app is a client of `libghostty` (`include/ghostty.h`). This port
+//! drives the *same* API surface in-process by calling the Rust functions of
+//! `ghostty-lib` (see [`ghostty::capi`]) with the same runtime configuration
+//! (wakeup, action, clipboard, close-surface callbacks), so behaviour matches
+//! the Swift app.
+//!
+//! Layout mirrors `Sources/`:
+//!
+//! | Swift                         | Rust                      |
+//! |-------------------------------|---------------------------|
+//! | `Sources/App/macOS`           | [`app`]                   |
+//! | `Sources/Ghostty`             | [`ghostty`]               |
+//! | `Sources/Helpers`             | [`helpers`]               |
+//! | `Sources/Features/<Area>`     | [`features`]`::<area>`    |
+//!
+//! Everything that touches AppKit is `cfg(target_vendor = "apple")`; on other
+//! targets the crate only contains the platform independent logic (config
+//! value parsing, shell quoting, menu tables, bundle metadata, ...) so it is
+//! tested on Linux and the workspace stays green. The AppKit code can only be
+//! compile-checked off macOS (`scripts/macos-crosscheck.sh`); see
+//! `docs/rewrite/notes/macos-app-design.md`.
+
+pub mod app;
+pub mod features;
+pub mod ghostty;
+pub mod helpers;