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/shape.rs

@@ -1,20 +1,29 @@
 //! Shaping types shared by all shapers. Port of `font/shape.zig`.
 //!
 //! The shaper implementation for the Linux backend is HarfBuzz
-//! ([`crate::shaper::harfbuzz::Shaper`]).
+//! (`crate::shaper::harfbuzz::Shaper`), for the macOS backend CoreText
+//! (`crate::shaper::coretext::Shaper`).
 
+#[cfg(font_coretext_shaper)]
+pub use crate::shaper::coretext;
+#[cfg(font_coretext_shaper)]
+pub use crate::shaper::coretext::{ShapeError, ShapeRunError, Shaper};
 pub use crate::shaper::feature::{DEFAULT_FEATURES, Feature, FeatureList};
+#[cfg(font_harfbuzz)]
+pub use crate::shaper::harfbuzz;
 /// Shaper implementation for our compile options (backend
-/// `fontconfig_freetype` => HarfBuzz).
-#[cfg(not(target_arch = "wasm32"))]
-pub use crate::shaper::harfbuzz::Shaper;
+/// `fontconfig_freetype`, `coretext_freetype`, `coretext_harfbuzz` =>
+/// HarfBuzz; `coretext` => CoreText; `coretext_noshape` => no-op).
+#[cfg(font_harfbuzz)]
+pub use crate::shaper::harfbuzz::{ShapeError, ShapeRunError, Shaper};
+#[cfg(font_noshape)]
+pub use crate::shaper::noop::{ShapeError, ShapeRunError, Shaper};
 /// Shaper implementation for the `web_canvas` backend (wasm32).
 #[cfg(target_arch = "wasm32")]
 pub use crate::shaper::web_canvas::Shaper;
 #[cfg(not(target_arch = "wasm32"))]
 pub use crate::shaper::{
     cache::Cache,
-    harfbuzz,
     noop,
     run::{RunIterator, RunIteratorHook, RunOptions, TextRun},
 };