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/Cargo.toml

@@ -11,12 +11,36 @@ thiserror = "2.0.21"
 
 # Zig's wasm build only compiles `os/wasm.zig`; the POSIX / D-Bus / gettext
 # dependencies are unavailable on wasm32.
-[target.'cfg(not(target_arch = "wasm32"))'.dependencies]
+[target.'cfg(not(any(target_arch = "wasm32", windows)))'.dependencies]
 gettext-rs = { version = "0.8.0", features = ["gettext-system"] }
 nix = { version = "0.31.3", features = ["user"] }
 rustix = { version = "1.1.5", features = ["fs", "process", "system", "pipe", "net", "time", "rand", "pty"] }
 zbus = "5.19.0"
 
+# Windows: ConPTY / pipes / process APIs (`os/windows.zig`).
+[target.'cfg(windows)'.dependencies]
+windows-sys = { version = "0.61", features = [
+    "Win32_Foundation",
+    "Win32_Security",
+    "Win32_Security_Cryptography",
+    "Win32_Storage_FileSystem",
+    "Win32_System_Console",
+    "Win32_System_IO",
+    "Win32_System_Pipes",
+    "Win32_System_WindowsProgramming",
+    "Win32_System_Threading",
+] }
+
+[target.'cfg(target_vendor = "apple")'.dependencies]
+libc = "0.2.190"
+objc2 = "0.6.5"
+objc2-app-kit = "0.3.2"
+objc2-core-foundation = "0.3.2"
+objc2-foundation = "0.3.2"
+
+[target.'cfg(not(target_arch = "wasm32"))'.dependencies]
+crossbeam-channel = "0.5.17"
+
 [features]
 # Mirrors Zig's `wasm_shared` build option (SharedArrayBuffer-backed memory).
 wasm-shared = []