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)

Cargo.toml

@@ -8,15 +8,26 @@ description = "A Rust rewrite of the Ghostty terminal emulator"
 name = "ghostty"
 path = "src/main.rs"
 
+[features]
+# Mirrors upstream's `-Dsentry` build option (off by default on Linux):
+# enables the local crash-report handler (`ghostty-crash`).
+sentry = ["ghostty-crash/sentry"]
+
 [dependencies]
 ghostty-core.workspace = true
 ghostty-config.workspace = true
 ghostty-apprt-gtk.workspace = true
 ghostty-apprt.workspace = true
 ghostty-cli.workspace = true
+ghostty-crash.workspace = true
 ghostty-os.workspace = true
 log = "0.4"
 
+[dev-dependencies]
+rustix = { version = "1", features = ["process"] }
+serde_json = "1"
+tempfile = "3"
+
 [lints]
 workspace = true
 
@@ -50,8 +61,11 @@ ghostty-core = { path = "crates/ghostty-core" }
 ghostty-apprt-gtk = { path = "crates/ghostty-apprt-gtk" }
 ghostty-apprt = { path = "crates/ghostty-apprt" }
 ghostty-cli = { path = "crates/ghostty-cli" }
+ghostty-crash = { path = "crates/ghostty-crash" }
 ghostty-bench = { path = "crates/ghostty-bench" }
 ghostty-inspector = { path = "crates/ghostty-inspector" }
+ghostty-vt-capi = { path = "crates/ghostty-vt-capi" }
+ghostty-wasm = { path = "crates/ghostty-wasm" }
 
 [workspace.lints.rust]
 unsafe_code = "forbid"