feat(apprt): port the core/runtime contract types

Ports ZIG/src/apprt/{action,structs,surface,ipc,runtime}.zig into
ghostty-apprt: the Action enum performed by the core on the app runtime
(all 65 variants with docs), Target<S> generic over the runtime's
surface handle, clipboard/size/scale/selection structs, the core->surface
Message enum with a Mailbox over an AppMailbox trait, and IPC action
types for +new-window.

Owned Strings and Arc<Config> replace Zig's borrowed slices/pointers.
Scrollbar and RendererHealth are local stand-ins until core integration.
The embedded C API and browser runtime are out of scope.

Test Plan:
- cargo test -p ghostty-apprt (16 tests)

crates/ghostty-apprt/src/runtime.rs

@@ -0,0 +1,61 @@
+//! Port of `runtime.zig` (and `none.zig`).
+
+/// Runtime is the runtime to use for Ghostty. All runtimes do not provide
+/// equivalent feature sets.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Runtime {
+    /// Will not produce an executable at all (lib only). Matches `none.zig`:
+    /// there is no apprt to communicate with, so IPC always returns `false`.
+    None,
+
+    /// GTK4. Rich windowed application. This uses a full GObject-based
+    /// approach to building the application.
+    Gtk,
+}
+
+impl Runtime {
+    /// The default runtime for the current target. The Linux and FreeBSD
+    /// default is GTK because it is a full featured application; otherwise
+    /// `None`.
+    #[must_use]
+    pub fn default_for_target() -> Self {
+        if cfg!(any(target_os = "linux", target_os = "freebsd")) {
+            Runtime::Gtk
+        } else {
+            Runtime::None
+        }
+    }
+}
+
+/// `none.zig` `App::performIpc`: always false as there is no apprt to
+/// communicate with.
+///
+/// # Errors
+/// Never fails.
+pub fn none_perform_ipc(
+    _target: &crate::ipc::Target,
+    _action: &crate::ipc::Action,
+) -> Result<bool, crate::ipc::Error> {
+    Ok(false)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn default_runtime_is_gtk_on_linux() {
+        if cfg!(target_os = "linux") {
+            assert_eq!(Runtime::default_for_target(), Runtime::Gtk);
+        }
+    }
+
+    #[test]
+    fn none_ipc_returns_false() {
+        let r = none_perform_ipc(
+            &crate::ipc::Target::Detect,
+            &crate::ipc::Action::NewWindow(crate::ipc::NewWindow { arguments: None }),
+        );
+        assert!(matches!(r, Ok(false)));
+    }
+}