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-config/src/edit.rs

@@ -9,8 +9,8 @@ use crate::file_load;
 /// On Linux, this will use the file at the XDG config path. This is the
 /// only valid path for Linux so we don't need to check for other paths.
 ///
-/// (On macOS both XDG and AppSupport paths are valid; macOS is not
-/// supported by this port.)
+/// On macOS, both XDG and AppSupport paths are valid; the AppSupport paths
+/// come first.
 ///
 /// The config file (and its directory) is created if it doesn't exist.
 pub fn open_path() -> io::Result<String> {
@@ -80,8 +80,15 @@ fn config_path() -> io::Result<String> {
 /// Returns a list of possible paths the main config file could be
 /// in for the current OS.
 fn config_path_candidates() -> io::Result<Vec<String>> {
-    Ok(vec![
-        file_load::default_xdg_path().map_err(xdg_err)?,
-        file_load::legacy_default_xdg_path().map_err(xdg_err)?,
-    ])
+    let mut paths = Vec::with_capacity(4);
+
+    #[cfg(target_os = "macos")]
+    {
+        paths.push(file_load::default_app_support_path().map_err(xdg_err)?);
+        paths.push(file_load::legacy_default_app_support_path().map_err(xdg_err)?);
+    }
+
+    paths.push(file_load::default_xdg_path().map_err(xdg_err)?);
+    paths.push(file_load::legacy_default_xdg_path().map_err(xdg_err)?);
+    Ok(paths)
 }