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-font/src/codepoint_resolver.rs

@@ -19,7 +19,7 @@ use crate::{
     Style,
     collection::{AddOptions, Collection, Entry, GetFaceError, Index, PresentationMode, Special},
     discovery::Discover,
-    face::{RenderOptions, freetype::RenderError as FaceRenderError},
+    face::{RenderError as FaceRenderError, RenderOptions},
     sprite,
 };
 
@@ -257,7 +257,7 @@ impl CodepointResolver {
             monospace: false,
             ..Default::default()
         };
-        let mut disco_it = disco.discover_fallback(&self.collection, &desc).ok()?;
+        let mut disco_it = disco.discover_fallback(&mut self.collection, &desc).ok()?;
 
         loop {
             let deferred_face = match disco_it.next() {
@@ -490,6 +490,8 @@ mod tests {
         c.load_options = Some(LoadOptions::new(lib.clone()));
 
         add(&mut c, &lib, embedded::REGULAR, Style::Regular, 12.0);
+        // Coretext doesn't support Noto's format
+        #[cfg(not(font_coretext))]
         add(&mut c, &lib, embedded::EMOJI, Style::Regular, 12.0);
         add(&mut c, &lib, embedded::EMOJI_TEXT, Style::Regular, 12.0);
 
@@ -515,7 +517,8 @@ mod tests {
                 .get_index(0x270C, Style::Regular, Some(Presentation::Text))
                 .unwrap();
             assert_eq!(Style::Regular, idx.style);
-            assert_eq!(2, idx.idx);
+            let text_idx = if cfg!(font_coretext) { 1 } else { 2 };
+            assert_eq!(text_idx, idx.idx);
         }
         {
             let idx = r