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/src/metal/sampler.rs

@@ -0,0 +1,53 @@
+//! Wrapper for handling samplers (`metal/Sampler.zig`).
+use objc2::{rc::Retained, runtime::ProtocolObject};
+use objc2_metal::{
+    MTLDevice as _,
+    MTLSamplerAddressMode,
+    MTLSamplerDescriptor,
+    MTLSamplerMinMagFilter,
+    MTLSamplerState,
+};
+
+use super::{Device, Error};
+
+/// Options for initializing a sampler.
+#[derive(Clone)]
+pub struct Options {
+    /// MTLDevice
+    pub device: Device,
+    pub min_filter: MTLSamplerMinMagFilter,
+    pub mag_filter: MTLSamplerMinMagFilter,
+    pub s_address_mode: MTLSamplerAddressMode,
+    pub t_address_mode: MTLSamplerAddressMode,
+}
+
+/// A sampler state.
+pub struct Sampler {
+    /// The underlying MTLSamplerState Object.
+    sampler: Retained<ProtocolObject<dyn MTLSamplerState>>,
+}
+
+impl Sampler {
+    /// The underlying MTLSamplerState.
+    pub fn sampler(&self) -> &ProtocolObject<dyn MTLSamplerState> {
+        &self.sampler
+    }
+
+    /// Initialize a sampler
+    pub fn init(opts: &Options) -> Result<Sampler, Error> {
+        // Create our descriptor and set the properties
+        let desc = MTLSamplerDescriptor::new();
+        desc.setMinFilter(opts.min_filter);
+        desc.setMagFilter(opts.mag_filter);
+        desc.setSAddressMode(opts.s_address_mode);
+        desc.setTAddressMode(opts.t_address_mode);
+
+        // Create the sampler state
+        let sampler = opts
+            .device
+            .newSamplerStateWithDescriptor(&desc)
+            .ok_or(Error::MetalFailed)?;
+
+        Ok(Sampler { sampler })
+    }
+}