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/args.rs

@@ -5,10 +5,28 @@
 //! don't panic).
 
 /// Returns an iterator over the command line arguments.
+#[cfg(not(target_os = "macos"))]
 pub fn iterator() -> std::env::ArgsOs {
     std::env::args_os()
 }
 
+/// Returns an iterator over the command line arguments.
+///
+/// On macOS this reads `NSProcessInfo.arguments` instead of libc
+/// argc/argv, which are not valid for `NSApplicationMain`-based
+/// applications (Zig: `IteratorMacOS`).
+#[cfg(target_os = "macos")]
+pub fn iterator() -> std::vec::IntoIter<std::ffi::OsString> {
+    use objc2_foundation::NSProcessInfo;
+
+    NSProcessInfo::processInfo()
+        .arguments()
+        .iter()
+        .map(|s| std::ffi::OsString::from(s.to_string()))
+        .collect::<Vec<_>>()
+        .into_iter()
+}
+
 /// Collect the process arguments as `String`s without ever panicking on
 /// non-UTF-8 bytes (invalid sequences become U+FFFD). The CLI/config layers
 /// are string based; `+action` dispatch only depends on the ASCII prefix.
@@ -30,6 +48,7 @@ mod tests {
         assert!(!iter.next().unwrap().is_empty());
     }
 
+    #[cfg(unix)]
     #[test]
     fn lossy_does_not_panic_on_non_utf8() {
         use std::os::unix::ffi::OsStringExt;