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/tests/egl_headless.rs

@@ -5,6 +5,8 @@
 //! Skipped (returns early with a message) when no EGL display / GL 4.3 core
 //! context can be created, e.g. on CI without Mesa.
 
+// These tests drive the OpenGL backend (Mesa); Apple platforms render with Metal.
+#![cfg(not(target_vendor = "apple"))]
 // The EGL loader calls are `unsafe fn`s in khronos-egl (test-only usage).
 #![allow(unsafe_code, clippy::unwrap_used)]
 
@@ -128,7 +130,7 @@ fn px(buf: &[u8], w: usize, x: usize, y: usize) -> [u8; 4] {
 
 #[test]
 fn headless_shaders_compile_and_render() {
-    let Some((_h, mut api)) = setup() else {
+    let Some((_h, api)) = setup() else {
         return;
     };
     let gl: Gl = api.gl().unwrap().clone();