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-renderer/Cargo.toml

@@ -8,6 +8,7 @@ license.workspace = true
 bytemuck = { version = "1.25.2", features = ["derive"] }
 crossbeam-channel = "0.5.17"
 ghostty-config.workspace = true
+ghostty-crash.workspace = true
 ghostty-datastruct.workspace = true
 ghostty-font.workspace = true
 ghostty-input.workspace = true
@@ -18,7 +19,7 @@ glow = "0.18.0"
 jpeg-decoder = { version = "0.3.2", default-features = false }
 khronos-egl = { version = "6.0.0", features = ["dynamic"] }
 log = "0.4.34"
-naga = { version = "30.0.1", features = ["glsl-in", "glsl-out", "wgsl-in"] }
+naga = { version = "30.0.1", features = ["glsl-in", "glsl-out", "msl-out", "wgsl-in"] }
 onig = "6.5.3"
 png = "0.18.1"
 thiserror = "2.0.21"
@@ -54,3 +55,21 @@ return_self_not_must_use = "allow"
 used_underscore_binding = "allow"
 large_stack_arrays = "allow"
 inline_always = "allow"
+
+# The Metal graphics API (`renderer/Metal.zig`) and the CoreVideo display
+# link; compiled on macOS and iOS only.
+[target.'cfg(target_vendor = "apple")'.dependencies]
+block2 = "0.6.2"
+dispatch2 = "0.3.1"
+ghostty-os.workspace = true
+objc2 = "0.6.5"
+objc2-core-foundation = "0.3.2"
+objc2-core-graphics = "0.3.2"
+objc2-foundation = "0.3.2"
+objc2-io-surface = "0.3.2"
+objc2-metal = { version = "0.3.2", features = ["block2", "objc2-io-surface"] }
+objc2-quartz-core = "0.3.2"
+
+# CVDisplayLink does not exist on iOS.
+[target.'cfg(target_os = "macos")'.dependencies]
+objc2-core-video = "0.3.2"