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-termio/src/lib.rs

@@ -1,6 +1,7 @@
 //! ghostty-termio: see docs/rewrite/ARCHITECTURE.md for the module map.
 //!
-//! Linux only. Ported: `pty.zig`, `Command.zig`, the `terminfo/*` source
+//! Linux, with a Windows (ConPTY) port behind `cfg(windows)` (the `*_windows.rs`
+//! files, selected with `#[path]`). Ported: `pty.zig`, `Command.zig`, the `terminfo/*` source
 //! generator and everything in `termio/` (`Options`, `backend`, `message`,
 //! `mailbox`, `Termio`, `Thread`, `Exec`, `stream_handler`,
 //! `shell_integration`). See `docs/rewrite/notes/ghostty-termio-IO1.md` and
@@ -19,11 +20,14 @@
 )]
 
 pub mod backend;
+#[cfg_attr(windows, path = "command_windows.rs")]
 pub mod command;
+#[cfg_attr(windows, path = "exec_windows.rs")]
 pub mod exec;
 pub mod mailbox;
 pub mod message;
 pub mod options;
+#[cfg_attr(windows, path = "pty_windows.rs")]
 pub mod pty;
 pub mod shell_integration;
 pub mod sinks;
@@ -32,7 +36,10 @@ pub mod stream_handler;
 pub mod terminfo;
 pub mod termio;
 pub mod thread;
+#[cfg_attr(windows, path = "wakeup_windows.rs")]
 pub mod wakeup;
+/// Pure Windows `CreateProcessW` input encoders (tested on every host).
+pub mod windows_args;
 
 pub use backend::Backend;
 pub use command::Command;