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/xdg.rs

@@ -21,6 +21,10 @@ pub struct Options<'a> {
 pub enum Error {
     #[error("no home directory could be determined")]
     NoHomeDir,
+    /// A macOS Foundation API (e.g. `NSFileManager`) failed (Zig:
+    /// `error.AppleAPIFailed`).
+    #[error("apple api failed")]
+    AppleApiFailed,
 }
 
 /// Get the XDG user config directory.
@@ -49,8 +53,12 @@ fn dir(opts: &Options<'_>, env: &str, default_subdir: &str) -> Result<String, Er
         ]));
     }
 
-    // First check the env var.
-    if let Some(env) = getenv_not_empty(env) {
+    // First check the env var. On Windows we treat `LOCALAPPDATA` as a
+    // fallback for the XDG variable.
+    let env_value = getenv_not_empty(env);
+    #[cfg(windows)]
+    let env_value = env_value.or_else(|| getenv_not_empty("LOCALAPPDATA"));
+    if let Some(env) = env_value {
         // If we have a subdir, then we join it.
         if let Some(subdir) = opts.subdir {
             return Ok(path_join(&[&env, subdir]));
@@ -90,7 +98,7 @@ pub fn parse_terminal_exec<'a>(argv: &'a [&'a CStr]) -> Option<&'a [&'a CStr]> {
     Some(&argv[start..])
 }
 
-#[cfg(test)]
+#[cfg(all(test, unix))]
 mod tests {
     use super::*;