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/app_info.rs

@@ -0,0 +1,34 @@
+//! `Helpers/AppInfo.swift`.
+
+/// True if we appear to be running in Xcode.
+#[must_use]
+pub fn is_running_in_xcode() -> bool {
+    is_running_in_xcode_with(|k| std::env::var_os(k).is_some())
+}
+
+fn is_running_in_xcode_with(has_var: impl Fn(&str) -> bool) -> bool {
+    has_var("__XCODE_BUILT_PRODUCTS_DIR_PATHS")
+}
+
+/// `Bundle.main.infoDictionary?[key] as? Bool`.
+#[cfg(target_vendor = "apple")]
+#[must_use]
+pub fn bundle_info_bool(key: &str) -> Option<bool> {
+    use objc2_foundation::{NSBundle, NSNumber, NSString};
+    let info = NSBundle::mainBundle().infoDictionary()?;
+    let value = info.objectForKey(&NSString::from_str(key))?;
+    value.downcast_ref::<NSNumber>().map(NSNumber::boolValue)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn detects_xcode_env() {
+        assert!(is_running_in_xcode_with(
+            |k| k == "__XCODE_BUILT_PRODUCTS_DIR_PATHS"
+        ));
+        assert!(!is_running_in_xcode_with(|_| false));
+    }
+}