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-core/src/surface_mouse.rs

@@ -103,13 +103,43 @@ fn is_mouse_mode_override_state(mods: Mods) -> bool {
 /// cause a rectangle select.
 #[must_use]
 pub fn is_rectangle_select_state(mods: Mods) -> bool {
-    mods.ctrl_or_super() && mods.alt
+    is_rectangle_select_state_for(mods, ghostty_input::platform::is_darwin())
+}
+
+/// [`is_rectangle_select_state`] for an explicit platform (Zig picks with
+/// `builtin.target.os.tag.isDarwin()` at comptime): on Darwin option alone
+/// selects a rectangle, elsewhere it is ctrl+alt.
+#[must_use]
+pub fn is_rectangle_select_state_for(mods: Mods, darwin: bool) -> bool {
+    if darwin {
+        mods.alt
+    } else {
+        mods.ctrl_or_super() && mods.alt
+    }
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
 
+    #[test]
+    fn rectangle_select_state_per_platform() {
+        let alt = Mods {
+            alt: true,
+            ..Mods::default()
+        };
+        let ctrl_alt = Mods {
+            alt: true,
+            ctrl: true,
+            ..Mods::default()
+        };
+        // macOS: alt alone.
+        assert!(is_rectangle_select_state_for(alt, true));
+        // Elsewhere: ctrl+alt.
+        assert!(!is_rectangle_select_state_for(alt, false));
+        assert!(is_rectangle_select_state_for(ctrl_alt, false));
+    }
+
     fn m(
         physical_key: Key,
         mouse_event: MouseEvents,