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-font/build.rs

@@ -0,0 +1,60 @@
+//! Emits the font backend `cfg`s (port of `font/backend.zig`'s `Backend`
+//! helpers `hasFreetype`/`hasCoretext`/`hasFontconfig`/`hasHarfbuzz`).
+//!
+//! Backends (see `src/backend.rs`):
+//! * Linux & other non-Apple, non-wasm targets: `fontconfig_freetype`.
+//! * wasm32: `web_canvas`.
+//! * Apple targets: `coretext` (default), or via cargo features
+//!   `coretext-freetype`, `coretext-harfbuzz`, `coretext-noshape`.
+//!
+//! Emitted cfgs:
+//! * `font_freetype`   - FreeType face / library are compiled.
+//! * `font_fontconfig` - fontconfig discovery is compiled.
+//! * `font_harfbuzz`   - the HarfBuzz shaper is compiled.
+//! * `font_coretext`   - CoreText discovery/deferred face (Apple targets).
+//! * `font_coretext_face` - the CoreText `Face` is the font face.
+//! * `font_coretext_shaper` - the CoreText shaper is the shaper.
+//! * `font_noshape`    - the no-op shaper is the shaper.
+
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+    for c in [
+        "font_freetype",
+        "font_fontconfig",
+        "font_harfbuzz",
+        "font_coretext",
+        "font_coretext_face",
+        "font_coretext_shaper",
+        "font_noshape",
+    ] {
+        println!("cargo:rustc-check-cfg=cfg({c})");
+    }
+
+    let arch = std::env::var("CARGO_CFG_TARGET_ARCH").unwrap_or_default();
+    let vendor = std::env::var("CARGO_CFG_TARGET_VENDOR").unwrap_or_default();
+    let feat = |n: &str| std::env::var_os(format!("CARGO_FEATURE_{n}")).is_some();
+
+    let wasm = arch == "wasm32";
+    let apple = vendor == "apple";
+    let ct_ft = feat("CORETEXT_FREETYPE");
+    let ct_hb = feat("CORETEXT_HARFBUZZ");
+    let ct_noshape = feat("CORETEXT_NOSHAPE");
+
+    let on = |c: &str, v: bool| {
+        if v {
+            println!("cargo:rustc-cfg={c}");
+        }
+    };
+
+    let freetype = !wasm && (!apple || ct_ft);
+    let harfbuzz = !wasm && (!apple || ct_ft || ct_hb);
+    on("font_freetype", freetype);
+    on("font_fontconfig", !wasm && !apple);
+    on("font_harfbuzz", harfbuzz);
+    on("font_coretext", apple);
+    on("font_coretext_face", apple && !ct_ft);
+    // The shaper is CoreText only for the plain `coretext` backend;
+    // `coretext_noshape` uses the no-op shaper.
+    on("font_coretext_shaper", apple && !harfbuzz && !ct_noshape);
+    on("font_noshape", apple && !harfbuzz && ct_noshape);
+}