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-renderer/src/lib.rs

@@ -14,16 +14,23 @@
 pub mod backend;
 pub mod cell;
 pub mod cursor;
+pub mod display_link;
 pub mod generic;
+pub mod graphics_api;
 pub mod headless;
 pub mod image;
 pub mod link;
 pub mod math;
 pub mod message;
+#[cfg(target_vendor = "apple")]
+#[allow(unsafe_code, clippy::struct_field_names)]
+pub mod metal;
+pub mod metal_layout;
 pub mod opengl;
 pub mod options;
 pub mod overlay;
 pub mod row;
+pub mod shader_types;
 pub mod shadertoy;
 mod shadertoy_sampler;
 pub mod size;
@@ -37,9 +44,19 @@ pub use opengl::OpenGL;
 pub use size::{CellSize, Coordinate, GridSize, Padding, ScreenSize, Size};
 pub use state::State;
 
-/// The renderer used on Linux: the generic renderer instantiated with the
-/// OpenGL graphics API (`renderer.zig`: `pub const Renderer = GenericRenderer(OpenGL)`).
-pub type Renderer = generic::Renderer;
+/// The graphics API the renderer is built with (`renderer.zig`: `Renderer =
+/// switch (build_config.renderer) { .metal => GenericRenderer(Metal), .opengl
+/// => GenericRenderer(OpenGL) }`): Metal on Apple platforms, OpenGL elsewhere
+/// (see [`Backend::default_for_target`]).
+#[cfg(target_vendor = "apple")]
+pub type Api = metal::Metal;
+/// The graphics API the renderer is built with: OpenGL.
+#[cfg(not(target_vendor = "apple"))]
+pub type Api = opengl::OpenGL;
+
+/// The renderer: the generic renderer instantiated with the graphics API
+/// selected for the target ([`Api`]).
+pub type Renderer = generic::Renderer<Api>;
 
 /// The renderer's derived configuration (`Renderer.DerivedConfig`).
 pub type DerivedConfig = generic::DerivedConfig;