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/structs.rs

@@ -0,0 +1,131 @@
+//! Port of `structs.zig`.
+
+/// ContentScale is the ratio between the current DPI and the platform's
+/// default DPI. This is used to determine how much certain rendered elements
+/// need to be scaled up or down.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct ContentScale {
+    pub x: f32,
+    pub y: f32,
+}
+
+/// The size of the surface in pixels.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct SurfaceSize {
+    pub width: u32,
+    pub height: u32,
+}
+
+/// The position of the cursor in pixels.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct CursorPos {
+    pub x: f32,
+    pub y: f32,
+}
+
+/// Input Method Editor (IME) position.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct ImePos {
+    pub x: f64,
+    pub y: f64,
+    pub width: f64,
+    pub height: f64,
+}
+
+/// The clipboard type.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+#[repr(u8)]
+pub enum Clipboard {
+    /// ctrl+c/v
+    Standard = 0,
+    Selection = 1,
+    Primary = 2,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ClipboardContent {
+    pub mime: String,
+    pub data: String,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ClipboardRequestType {
+    Paste,
+    Osc52Read,
+    Osc52Write,
+}
+
+/// Clipboard request. This is used to request clipboard contents and must
+/// be sent as a response to a ClipboardRequest event.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ClipboardRequest {
+    /// A direct paste of clipboard contents.
+    Paste,
+
+    /// A request to read clipboard contents via OSC 52.
+    Osc52Read(Clipboard),
+
+    /// A request to write clipboard contents via OSC 52.
+    Osc52Write(Clipboard),
+}
+
+impl ClipboardRequest {
+    #[must_use]
+    pub fn request_type(&self) -> ClipboardRequestType {
+        match self {
+            Self::Paste => ClipboardRequestType::Paste,
+            Self::Osc52Read(_) => ClipboardRequestType::Osc52Read,
+            Self::Osc52Write(_) => ClipboardRequestType::Osc52Write,
+        }
+    }
+}
+
+/// The color scheme in use (light vs dark).
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[repr(u8)]
+pub enum ColorScheme {
+    Light = 0,
+    Dark = 1,
+}
+
+/// Selection information
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct Selection {
+    /// Top-left point of the selection in the viewport in scaled
+    /// window pixels. (0,0) is the top-left of the window.
+    pub tl_x_px: f64,
+    pub tl_y_px: f64,
+
+    /// The offset of the selection start in cells from the top-left
+    /// of the viewport.
+    ///
+    /// This is a strange metric but its used by macOS.
+    pub offset_start: u32,
+    pub offset_len: u32,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn enum_values() {
+        assert_eq!(Clipboard::Standard as u8, 0);
+        assert_eq!(Clipboard::Selection as u8, 1);
+        assert_eq!(Clipboard::Primary as u8, 2);
+        assert_eq!(ColorScheme::Light as u8, 0);
+        assert_eq!(ColorScheme::Dark as u8, 1);
+    }
+
+    #[test]
+    fn clipboard_request_type() {
+        assert_eq!(
+            ClipboardRequest::Osc52Read(Clipboard::Primary).request_type(),
+            ClipboardRequestType::Osc52Read
+        );
+        assert_eq!(
+            ClipboardRequest::Paste.request_type(),
+            ClipboardRequestType::Paste
+        );
+    }
+}