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-os/src/hostname.rs

@@ -84,12 +84,22 @@ pub fn is_valid(hostname: &str) -> bool {
     true
 }
 
+/// The current machine's hostname (`GetComputerNameA`).
+#[cfg(windows)]
+fn our_hostname() -> Option<String> {
+    crate::windows::computer_name()
+}
+
 /// The current machine's hostname (`gethostname`).
-fn our_hostname() -> String {
-    rustix::system::uname()
-        .nodename()
-        .to_string_lossy()
-        .into_owned()
+#[cfg(unix)]
+#[allow(clippy::unnecessary_wraps)]
+fn our_hostname() -> Option<String> {
+    Some(
+        rustix::system::uname()
+            .nodename()
+            .to_string_lossy()
+            .into_owned(),
+    )
 }
 
 /// Checks if a hostname is local to the current machine. This matches
@@ -102,7 +112,7 @@ pub fn is_local(hostname: &str) -> Result<bool, LocalHostnameValidationError> {
     }
 
     // If hostname is not "localhost" it must match our hostname.
-    Ok(hostname == our_hostname())
+    Ok(our_hostname().is_some_and(|ours| hostname == ours))
 }
 
 #[cfg(test)]
@@ -147,7 +157,7 @@ mod tests {
 
     #[test]
     fn is_local_returns_true_when_hostname_is_local() {
-        let local = our_hostname();
+        let local = our_hostname().unwrap();
         assert!(is_local(&local).unwrap());
     }