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-terminal/src/search/thread.rs

@@ -234,6 +234,18 @@ impl Thread {
     pub fn thread_main(mut self) {
         log::debug!("starting search thread");
 
+        // On Darwin we name the thread explicitly and run with a lower
+        // priority than other threads.
+        #[cfg(target_vendor = "apple")]
+        {
+            ghostty_os::macos::pthread_setname_np(c"search");
+            let class = ghostty_os::macos::QosClass::Utility;
+            match ghostty_os::macos::set_qos_class(class) {
+                Ok(()) => log::debug!("thread QoS class set class={class:?}"),
+                Err(err) => log::warn!("error setting QoS class err={err}"),
+            }
+        }
+
         // Drain the mailbox immediately (Zig sends an initial wakeup).
         self.drain_mailbox();
         self.next_refresh = Instant::now() + REFRESH_INTERVAL;