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-input/src/lib.rs

@@ -10,15 +10,24 @@ pub mod key_encode;
 pub mod key_mods;
 pub mod keyboard;
 pub mod keycodes;
+pub mod keymap_darwin;
+pub mod keymap_noop;
 pub mod kitty;
 pub mod link;
 pub mod mouse;
 pub mod paste;
+pub mod platform;
 
 pub use config::OptionAsAlt;
 pub use key::{Action, Key, KeyEvent, ctrl_or_super};
 pub use key_mods::{Mods, RemapSet as KeyRemapSet};
 pub use keyboard::Layout as KeyboardLayout;
+/// The platform keymap: Carbon `UCKeyTranslate` on macOS, a no-op elsewhere
+/// (Zig: `input.Keymap`).
+#[cfg(target_os = "macos")]
+pub use keymap_darwin::{Keymap, State as KeymapState, Translation as KeymapTranslation};
+#[cfg(not(target_os = "macos"))]
+pub use keymap_noop::{Keymap, State as KeymapState, Translation as KeymapTranslation};
 pub use link::Link;
 pub use mouse::{
     Button as MouseButton,