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)

justfile

@@ -25,6 +25,19 @@ _fix:
 
 fix: _fix fmt
 
+# Build libghostty-vt (C ABI, include/ghostty/vt.h) as target/<profile>/libghostty-vt.{so,a}
+# and, if the target is installed, target/wasm32-unknown-unknown/<profile>/ghostty-vt.wasm.
+vt-lib profile="release":
+    cargo build -p ghostty-vt-capi --profile {{profile}}
+    cp target/{{profile}}/libghostty_vt_capi.so target/{{profile}}/libghostty-vt.so
+    cp target/{{profile}}/libghostty_vt_capi.a target/{{profile}}/libghostty-vt.a
+    cargo build -p ghostty-vt-capi --profile {{profile}} --target wasm32-unknown-unknown
+    cp target/wasm32-unknown-unknown/{{profile}}/ghostty_vt_capi.wasm target/wasm32-unknown-unknown/{{profile}}/ghostty-vt.wasm
+
+# Run the libghostty-vt ABI check and the C/wasm example tests.
+vt-test:
+    cargo test -p ghostty-vt-capi
+
 clippy:
     cargo clippy --workspace --all-targets --all-features -- -D warnings