feat(macos): port Ghostty's macOS code paths (Metal, CoreText, Darwin)

Ports the macOS-specific Zig code behind cfg(target_os = "macos"):
- renderer: a GraphicsApi trait with generic::Renderer<G>; Metal backend
  (all renderer/metal/* files, shaders.metal verbatim, runtime library
  compile, IOSurface layer, CVDisplayLink), shadertoy -> MSL via naga;
  OpenGL remains the backend elsewhere.
- font: CoreText face, shaper and discovery with build.rs backend
  selection (coretext default; coretext-freetype / coretext-harfbuzz /
  coretext-noshape features), no fontconfig/freetype/harfbuzz on Apple.
- input: KeymapDarwin via UCKeyTranslate, option-as-alt, mac keycodes.
- os: macos, mach, cf_release_thread, NSProcessInfo args and macOS
  branches; config: macOS defaults, keybinds, Application Support paths.
- termio/core: login(1) wrapping, openpty/execvpe, Darwin child exit
  handling, os_log logging.

Compile-checked for aarch64/x86_64-apple-darwin (clippy -D warnings
clean) using the cross setup in docs/rewrite/notes/macos-M1.md; cannot
run here. The embedded runtime/C API (embedded.zig) is the next step.

Also includes the cross-platform integration edits shared by the crash,
verification, Windows and macOS work (Cargo target-gating, thread
crash-context hooks, apprt-gtk surface/IME refactor, termio/os cfg
gates), a cfg fix so ghostty-config builds on wasm32, and an allow for
the intentional recursion in the crash overflow test.

Test Plan:
- cargo test --workspace (3224 passed)
- cargo clippy --workspace --all-targets -- -D warnings
- cargo check --target aarch64-apple-darwin -p ghostty-renderer (with
  the documented cross env)

crates/ghostty-os/src/systemd.rs

@@ -1,12 +1,15 @@
+#[cfg(any(target_os = "linux", target_os = "android"))]
 use std::io::Read;
 
 /// Kernel `TASK_COMM_LEN`.
+#[cfg(any(target_os = "linux", target_os = "android"))]
 const TASK_COMM_LEN: u64 = 16;
 
 /// Returns true if the program was launched as a systemd service.
 ///
 /// On Linux, this returns true if the program was launched as a systemd
 /// service. It will return false if Ghostty was launched any other way.
+#[cfg(any(target_os = "linux", target_os = "android"))]
 pub fn launched_by_systemd() -> bool {
     // On Linux, systemd sets the `INVOCATION_ID` (v232+) and the
     // `JOURNAL_STREAM` (v231+) environment variables. If these
@@ -58,6 +61,12 @@ pub fn launched_by_systemd() -> bool {
     &comm_data[..end] == b"systemd"
 }
 
+/// For other platforms (macOS, ...), this returns false.
+#[cfg(not(any(target_os = "linux", target_os = "android")))]
+pub fn launched_by_systemd() -> bool {
+    false
+}
+
 /// systemd notifications. Used by Ghostty to inform systemd of the state of the
 /// process. Currently only used to notify systemd that we are ready and that
 /// configuration reloading has started.
@@ -68,6 +77,7 @@ pub fn launched_by_systemd() -> bool {
 /// avoid another external dependency. Linux systems that do not use
 /// `systemd` should never start Ghostty with the `NOTIFY_SOCKET` environment
 /// variable set and these functions essentially become a no-op.
+#[cfg(any(target_os = "linux", target_os = "android"))]
 pub mod notify {
     use rustix::net::{
         AddressFamily,
@@ -172,8 +182,17 @@ pub mod notify {
     }
 }
 
+/// systemd notifications are a no-op on platforms without systemd.
+#[cfg(not(any(target_os = "linux", target_os = "android")))]
+pub mod notify {
+    pub fn ready() {}
+
+    pub fn reloading() {}
+}
+
 #[cfg(test)]
 mod tests {
+    #[cfg(any(target_os = "linux", target_os = "android"))]
     use std::os::unix::net::UnixDatagram;
 
     use super::*;
@@ -190,6 +209,7 @@ mod tests {
         );
     }
 
+    #[cfg(any(target_os = "linux", target_os = "android"))]
     fn recv_with_socket(f: impl FnOnce()) -> String {
         let dir = crate::TempDir::init().unwrap();
         let path = dir.path().join("notify.sock");
@@ -202,11 +222,13 @@ mod tests {
         String::from_utf8(buf[..n].to_vec()).unwrap()
     }
 
+    #[cfg(any(target_os = "linux", target_os = "android"))]
     #[test]
     fn notify_ready_sends_message() {
         assert_eq!(recv_with_socket(notify::ready), "READY=1");
     }
 
+    #[cfg(any(target_os = "linux", target_os = "android"))]
     #[test]
     fn notify_reloading_sends_message() {
         let msg = recv_with_socket(notify::reloading);