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/features/custom_app_icon/mod.rs

@@ -0,0 +1,28 @@
+//! `Sources/Features/Custom App Icon` (owner: MA4).
+//! MA1 provides only the notification name and the `AppDelegate.updateAppIcon`
+//! hook. TODO(MA4): AppIcon, ColorizedGhosttyIcon, DockTilePlugin, UserDefaults+AppIcon.
+
+use crate::helpers::notification_center::Name;
+
+/// `Notification.Name.ghosttyIconDidChange` (posted on the *distributed*
+/// notification center).
+pub const GHOSTTY_ICON_DID_CHANGE: Name = Name("com.mitchellh.ghostty.iconDidChange");
+
+/// Posts `.ghosttyIconDidChange` on the distributed notification center
+/// (`DistributedNotificationCenter.default().postNotificationName(...,
+/// deliverImmediately: true)`), which the dock tile plugin observes.
+#[cfg(target_vendor = "apple")]
+#[allow(unsafe_code)]
+pub fn post_icon_did_change() {
+    use objc2_foundation::{NSDistributedNotificationCenter, NSString};
+    let center = NSDistributedNotificationCenter::defaultCenter();
+    // SAFETY: a name string, no object/userInfo.
+    unsafe {
+        center.postNotificationName_object_userInfo_deliverImmediately(
+            &NSString::from_str(GHOSTTY_ICON_DID_CHANGE.raw_value()),
+            None,
+            None,
+            true,
+        );
+    }
+}