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

Cargo.lock

@@ -1611,6 +1611,27 @@ dependencies = [
  "tempfile",
 ]
 
+[[package]]
+name = "ghostty-macos"
+version = "0.1.0"
+dependencies = [
+ "bitflags 2.13.2",
+ "block2",
+ "dispatch2",
+ "ghostty-lib",
+ "libc",
+ "log",
+ "objc2",
+ "objc2-app-kit",
+ "objc2-core-foundation",
+ "objc2-core-graphics",
+ "objc2-foundation",
+ "objc2-metal",
+ "objc2-quartz-core",
+ "objc2-user-notifications",
+ "tempfile",
+]
+
 [[package]]
 name = "ghostty-onig"
 version = "0.1.0"
@@ -2770,6 +2791,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "08849bbd4767dfae9457696856ae1c84fe4e0281bbe4a7abff2d0e06fb7981f8"
 dependencies = [
  "objc2-encode",
+ "objc2-exception-helper",
 ]
 
 [[package]]
@@ -2854,6 +2876,16 @@ dependencies = [
  "objc2-foundation",
 ]
 
+[[package]]
+name = "objc2-core-location"
+version = "0.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ca347214e24bc973fc025fd0d36ebb179ff30536ed1f80252706db19ee452009"
+dependencies = [
+ "objc2",
+ "objc2-foundation",
+]
+
 [[package]]
 name = "objc2-core-text"
 version = "0.3.2"
@@ -2889,6 +2921,15 @@ version = "4.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ef25abbcd74fb2609453eb695bd2f860d389e457f67dc17cafc8b8cbc89d0c33"
 
+[[package]]
+name = "objc2-exception-helper"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c7a1c5fbb72d7735b076bb47b578523aedc40f3c439bea6dfd595c089d79d98a"
+dependencies = [
+ "cc",
+]
+
 [[package]]
 name = "objc2-foundation"
 version = "0.3.2"
@@ -2947,6 +2988,19 @@ dependencies = [
  "objc2-metal",
 ]
 
+[[package]]
+name = "objc2-user-notifications"
+version = "0.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9df9128cbbfef73cda168416ccf7f837b62737d748333bfe9ab71c245d76613e"
+dependencies = [
+ "bitflags 2.13.2",
+ "block2",
+ "objc2",
+ "objc2-core-location",
+ "objc2-foundation",
+]
+
 [[package]]
 name = "objc_id"
 version = "0.1.1"