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/i18n.rs

@@ -1,3 +1,4 @@
+#[cfg(not(windows))]
 use std::path::Path;
 
 use crate::i18n_locales::LOCALES;
@@ -27,6 +28,15 @@ pub enum InitError {
 /// of translations. This does NOT call `textdomain` as we don't
 /// want to set the domain for the entire application since this is also
 /// used by libghostty.
+#[cfg(windows)]
+#[allow(clippy::unnecessary_wraps)]
+pub fn init(_resources_dir: &str) -> Result<(), InitError> {
+    // i18n is unsupported on Windows
+    Ok(())
+}
+
+/// See the Windows variant above.
+#[cfg(not(windows))]
 pub fn init(resources_dir: &str) -> Result<(), InitError> {
     // Our resources dir is always nested below the share dir that
     // is standard for translations.
@@ -52,12 +62,16 @@ pub fn init(resources_dir: &str) -> Result<(), InitError> {
 /// This should only be called for apprts that are fully owning the
 /// Ghostty application. This should not be called for libghostty users.
 pub fn init_global_domain() -> Result<(), InitError> {
+    #[cfg(not(windows))]
     gettextrs::textdomain(BUNDLE_ID).map_err(|_| InitError::OutOfMemory)?;
     Ok(())
 }
 
 /// Translate a message for the Ghostty domain (Zig: `_`).
 pub fn tr(msgid: &str) -> String {
+    #[cfg(windows)]
+    return msgid.to_owned();
+    #[cfg(not(windows))]
     gettextrs::dgettext(BUNDLE_ID, msgid)
 }