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/duration.rs

@@ -0,0 +1,25 @@
+//! `Helpers/Extensions/Duration+Extension.swift`.
+
+use std::time::Duration;
+
+pub trait DurationExt {
+    /// `Duration.timeInterval` (seconds as a double).
+    fn time_interval(&self) -> f64;
+}
+
+impl DurationExt for Duration {
+    fn time_interval(&self) -> f64 {
+        self.as_secs() as f64 + f64::from(self.subsec_nanos()) / 1_000_000_000.0
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn seconds() {
+        assert_eq!(Duration::from_millis(1500).time_interval(), 1.5);
+        assert_eq!(Duration::ZERO.time_interval(), 0.0);
+    }
+}