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/backend.rs

@@ -9,8 +9,10 @@ pub enum Backend {
 }
 
 impl Backend {
-    /// The default backend for the target. Metal/WebGL are out of scope for
-    /// the Rust port (Linux only), so this is always OpenGL on non-Darwin.
+    /// The default backend for the target (Metal on Apple platforms, OpenGL
+    /// otherwise). WebGL is only a name here: the wasm build has no GL renderer
+    /// in this crate. This is what [`crate::Api`] / [`crate::Renderer`] are
+    /// instantiated with.
     pub fn default_for_target() -> Backend {
         if cfg!(target_vendor = "apple") {
             Backend::Metal
@@ -27,3 +29,25 @@ pub enum Health {
     Healthy,
     Unhealthy,
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn default_backend_matches_selected_api() {
+        // `renderer.zig` picks the graphics API from the backend.
+        if cfg!(target_vendor = "apple") {
+            assert_eq!(Backend::default_for_target(), Backend::Metal);
+        } else {
+            assert_eq!(Backend::default_for_target(), Backend::Opengl);
+        }
+    }
+
+    #[test]
+    fn health_values_match_ghostty_h() {
+        // `GHOSTTY_RENDERER_HEALTH_{OK,ERROR}`.
+        assert_eq!(Health::Healthy as i32, 0);
+        assert_eq!(Health::Unhealthy as i32, 1);
+    }
+}