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-font/src/lib.rs

@@ -1,7 +1,9 @@
-//! Font engine: metrics, atlas, sprite font, faces (FreeType), discovery
-//! (fontconfig), collections, codepoint resolution, shaping (HarfBuzz) and
-//! the shared glyph grid. Port of `ZIG/src/font/` for the Linux backend
-//! (`fontconfig_freetype`).
+//! Font engine: metrics, atlas, sprite font, faces (FreeType / CoreText),
+//! discovery (fontconfig / CoreText), collections, codepoint resolution,
+//! shaping (HarfBuzz / CoreText) and the shared glyph grid. Port of
+//! `ZIG/src/font/`. The backend is selected per target (see `backend.rs` and
+//! `build.rs`): `fontconfig_freetype` on Linux, `coretext` on Apple targets,
+//! `web_canvas` on wasm32.
 
 #![cfg_attr(test, allow(clippy::unwrap_used))]
 
@@ -13,7 +15,12 @@ pub mod codepoint_resolver;
 #[cfg(not(target_arch = "wasm32"))]
 pub mod collection;
 pub mod config_types;
-#[cfg(not(target_arch = "wasm32"))]
+#[cfg(font_coretext)]
+pub mod coretext_util;
+#[cfg(font_fontconfig)]
+pub mod deferred_face;
+#[cfg(font_coretext)]
+#[path = "deferred_face_coretext.rs"]
 pub mod deferred_face;
 #[cfg(target_arch = "wasm32")]
 #[path = "deferred_face_web_canvas.rs"]
@@ -23,7 +30,7 @@ pub mod discovery;
 #[cfg(not(target_arch = "wasm32"))]
 pub mod embedded;
 pub mod face;
-#[cfg(not(target_arch = "wasm32"))]
+#[cfg(font_fontconfig)]
 pub mod fontconfig;
 pub mod glyph;
 #[cfg(not(target_arch = "wasm32"))]