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/ns_appearance.rs

@@ -0,0 +1,41 @@
+//! `Helpers/Extensions/NSAppearance+Extension.swift`.
+
+use objc2::rc::Retained;
+use objc2_app_kit::{NSAppearance, NSAppearanceNameAqua, NSAppearanceNameDarkAqua};
+
+use super::os_color;
+use crate::ghostty::config::Config;
+
+pub trait NSAppearanceExt {
+    /// Returns true if the appearance is some kind of dark.
+    fn is_dark(&self) -> bool;
+}
+
+impl NSAppearanceExt for NSAppearance {
+    fn is_dark(&self) -> bool {
+        self.name().to_string().to_lowercase().contains("dark")
+    }
+}
+
+/// `NSAppearance(ghosttyConfig:)`: the desired NSAppearance for the Ghostty
+/// configuration (`window-theme`), if any.
+#[must_use]
+#[allow(unsafe_code)]
+pub fn appearance_from_ghostty_config(config: &Config) -> Option<Retained<NSAppearance>> {
+    let theme = config.window_theme()?;
+    // SAFETY: reading AppKit's immutable appearance name constants.
+    let (aqua, dark) = unsafe { (NSAppearanceNameAqua, NSAppearanceNameDarkAqua) };
+    match theme.as_str() {
+        "dark" => NSAppearance::appearanceNamed(dark),
+        "light" => NSAppearance::appearanceNamed(aqua),
+        "auto" => {
+            let c = config.background_color();
+            if os_color::is_light_color(c.red, c.green, c.blue) {
+                NSAppearance::appearanceNamed(aqua)
+            } else {
+                NSAppearance::appearanceNamed(dark)
+            }
+        }
+        _ => None,
+    }
+}