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/open.rs

@@ -25,9 +25,7 @@ pub enum OpenUrlKind {
 /// apprt for Ghostty, you should consider doing something special-cased
 /// for your platform.
 pub fn open(kind: OpenUrlKind, url: &str) -> io::Result<()> {
-    let _ = kind;
-    let mut cmd = Command::new("xdg-open");
-    cmd.arg(url);
+    let mut cmd = open_command(kind, url)?;
 
     // Pipe stdout/stderr so we can collect output from the command.
     cmd.stdin(Stdio::null())
@@ -57,6 +55,52 @@ pub fn open(kind: OpenUrlKind, url: &str) -> io::Result<()> {
     Ok(())
 }
 
+/// Build the platform's "open URL" command (Zig `open`'s switch over
+/// `builtin.os.tag`).
+#[cfg(windows)]
+#[allow(clippy::unnecessary_wraps)]
+fn open_command(kind: OpenUrlKind, url: &str) -> io::Result<Command> {
+    let _ = kind;
+    let mut cmd = Command::new("rundll32");
+    cmd.args(["url.dll,FileProtocolHandler", url]);
+    Ok(cmd)
+}
+
+/// Linux and the BSDs use `xdg-open`.
+#[cfg(not(any(windows, target_os = "macos", target_os = "ios")))]
+#[allow(clippy::unnecessary_wraps)]
+fn open_command(kind: OpenUrlKind, url: &str) -> io::Result<Command> {
+    let _ = kind;
+    let mut cmd = Command::new("xdg-open");
+    cmd.arg(url);
+    Ok(cmd)
+}
+
+/// macOS uses `open` (`open -t` for text files).
+#[cfg(target_os = "macos")]
+#[allow(clippy::unnecessary_wraps)]
+fn open_command(kind: OpenUrlKind, url: &str) -> io::Result<Command> {
+    let mut cmd = Command::new("open");
+    cmd.args(macos_open_args(kind, url));
+    Ok(cmd)
+}
+
+/// Arguments of the macOS `open` command for a URL kind.
+#[cfg_attr(not(any(target_os = "macos", all(test, unix))), allow(dead_code))]
+fn macos_open_args(kind: OpenUrlKind, url: &str) -> Vec<&str> {
+    match kind {
+        OpenUrlKind::Text => vec!["-t", url],
+        OpenUrlKind::Html | OpenUrlKind::Unknown => vec![url],
+    }
+}
+
+/// iOS cannot spawn processes (`error.Unimplemented`).
+#[cfg(target_os = "ios")]
+#[allow(clippy::unnecessary_wraps)]
+fn open_command(_kind: OpenUrlKind, _url: &str) -> io::Result<Command> {
+    Err(io::Error::new(io::ErrorKind::Unsupported, "unimplemented"))
+}
+
 fn open_thread(child: std::process::Child) {
     // 50 KiB is the default value used by std.process.Child.run and should
     // be enough to get the output we care about.
@@ -78,10 +122,18 @@ fn open_thread(child: std::process::Child) {
     }
 }
 
-#[cfg(test)]
+#[cfg(all(test, unix))]
 mod tests {
     use super::*;
 
+    #[test]
+    fn macos_open_command_args() {
+        assert_eq!(macos_open_args(OpenUrlKind::Text, "/a"), ["-t", "/a"]);
+        assert_eq!(macos_open_args(OpenUrlKind::Html, "u"), ["u"]);
+        assert_eq!(macos_open_args(OpenUrlKind::Unknown, "u"), ["u"]);
+    }
+
+    #[cfg(not(target_vendor = "apple"))]
     #[test]
     fn open_reports_spawn_failure_quickly() {
         // With an empty PATH, xdg-open can't be found; Zig detects this