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/key.rs

@@ -538,7 +538,9 @@ impl Key {
     /// true if this key is one of the left or right versions of super (MacOS)
     /// or ctrl.
     pub fn ctrl_or_super(self) -> bool {
-        // Non-darwin target.
+        if crate::platform::is_darwin() {
+            return self == Key::MetaLeft || self == Key::MetaRight;
+        }
         self == Key::ControlLeft || self == Key::ControlRight
     }
 
@@ -558,9 +560,18 @@ impl Key {
 /// Mac, we default to super (i.e. super+c for copy) and on
 /// non-Mac we default to ctrl (i.e. ctrl+c for copy).
 pub fn ctrl_or_super(mods: Mods) -> Mods {
+    ctrl_or_super_for(mods, crate::platform::is_darwin())
+}
+
+/// [`ctrl_or_super`] for an explicit platform (`darwin == true` sets super).
+pub fn ctrl_or_super_for(mods: Mods, darwin: bool) -> Mods {
     let mut copy = mods;
-    // Non-darwin target.
-    copy.ctrl = true;
+    if darwin {
+        copy.super_ = true;
+    } else {
+        copy.ctrl = true;
+    }
+
     copy
 }
 
@@ -604,6 +615,23 @@ mod tests {
         assert!(Key::Unidentified.should_be_remappable());
     }
 
+    #[test]
+    fn key_ctrl_or_super_follows_platform() {
+        crate::platform::with_darwin(true, || {
+            assert!(Key::MetaLeft.ctrl_or_super());
+            assert!(Key::MetaRight.ctrl_or_super());
+            assert!(!Key::ControlLeft.ctrl_or_super());
+            assert!(ctrl_or_super(Mods::default()).super_);
+            assert!(!ctrl_or_super(Mods::default()).ctrl);
+        });
+        crate::platform::with_darwin(false, || {
+            assert!(Key::ControlLeft.ctrl_or_super());
+            assert!(!Key::MetaLeft.ctrl_or_super());
+            assert!(ctrl_or_super(Mods::default()).ctrl);
+            assert!(!ctrl_or_super(Mods::default()).super_);
+        });
+    }
+
     #[test]
     fn ctrl_or_super_test() {
         let m = ctrl_or_super(Mods::default());