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-termio/Cargo.toml

@@ -8,16 +8,32 @@ license.workspace = true
 crossbeam-channel = "0.5.17"
 ghostty-apprt.workspace = true
 ghostty-config.workspace = true
+ghostty-crash.workspace = true
 ghostty-datastruct.workspace = true
 ghostty-os.workspace = true
 ghostty-renderer.workspace = true
 ghostty-terminal.workspace = true
 ghostty-vt.workspace = true
-libc = "0.2.190"
 log = "0.4.34"
+thiserror = "2.0.21"
+
+[target.'cfg(unix)'.dependencies]
+libc = "0.2.190"
 nix = { version = "0.31.3", features = ["signal"] }
 rustix = { version = "1.1.5", features = ["fs", "process", "pty", "termios", "stdio", "event", "pipe"] }
-thiserror = "2.0.21"
+
+# ConPTY, pipes, process creation and waits (Zig: `pty.zig` WindowsPty,
+# `Command.zig` startWindows, `termio/Exec.zig` Windows branches).
+[target.'cfg(windows)'.dependencies]
+windows-sys = { version = "0.61", features = [
+    "Win32_Foundation",
+    "Win32_Security",
+    "Win32_Storage_FileSystem",
+    "Win32_System_Console",
+    "Win32_System_IO",
+    "Win32_System_Pipes",
+    "Win32_System_Threading",
+] }
 
 # Termio forks/execs children and sets up ptys; unsafe is denied by default
 # and only allowed on minimal, SAFETY-commented items.
@@ -55,3 +71,11 @@ inline_always = "allow"
 
 [dev-dependencies]
 temp-env = "0.3.6"
+
+[target.'cfg(unix)'.dev-dependencies]
+zbus = "5.19.0"
+rustix = { version = "1.1.5", features = ["process"] }
+
+[target.'cfg(target_vendor = "apple")'.dependencies]
+objc2-core-foundation = "0.3.2"
+objc2-foundation = "0.3.2"