feat(lib): port libghostty, Ghostty's embedded C API

Ports apprt/embedded.zig, main_c.zig and config/CApi.zig + c_get.zig as
crates/ghostty-lib (cdylib/staticlib/rlib `libghostty`), exporting every
function of the unchanged include/ghostty.h - the ABI Ghostty's Swift
macOS/iOS app links against. The embedded runtime implements the
runtime traits over the host's C callbacks, so the real core drives it.

- ghostty-core selects the renderer per target: Metal on Apple (surface
  NSView/UIView via RtSurface::platform_view), OpenGL elsewhere.
- ghostty_config_get follows Zig's c_get rules via a CValue type (143
  supported keys, checked against the Zig field types).
- The inspector C API is complete; its Metal backend replaces ImGui's
  Metal renderer and has not run.

Verification: generated C ABI probe (sizeof/alignof/offsetof of every
struct/union, 755 enum values, every function linked, nm exports, headers
byte-identical to upstream); a C host on Linux covering init, CLI
actions, config, app, keys, focus and color scheme; an opt-in
test-platform feature creating real surfaces on Linux with headless EGL
and a pty (13 tests). Apple targets are clippy-clean but not run; the
Swift app has not been built against it.

Test Plan:
- cargo test -p ghostty-lib (and --features test-platform)
- cargo clippy --target aarch64-apple-darwin -p ghostty-lib (cross env
  in docs/rewrite/notes/macos-M1.md)

crates/ghostty-core/src/app.rs

@@ -488,6 +488,27 @@ impl App {
         }
     }
 
+    /// Create a surface and add it to the app (`CoreApp::new_surface` with
+    /// the concrete surface type, for runtimes that need more than the
+    /// [`CoreSurface`] trait, e.g. the embedded C API).
+    pub fn create_surface(
+        &self,
+        id: SurfaceId,
+        config: Config,
+        rt_app: Rc<dyn RtApp>,
+        rt_surface: Rc<dyn RtSurface>,
+    ) -> Result<Rc<Surface>, CoreError> {
+        let surface = Surface::new(
+            id,
+            Rc::clone(&self.state),
+            Arc::new(config),
+            Rc::clone(&rt_app),
+            rt_surface,
+        )?;
+        self.add_surface(id, &surface, &rt_app);
+        Ok(surface)
+    }
+
     /// Add an initialized surface. Since we have non-zero surfaces, the quit
     /// timer is cancelled.
     pub(crate) fn add_surface(&self, id: SurfaceId, surface: &Rc<Surface>, rt: &Rc<dyn RtApp>) {
@@ -589,15 +610,7 @@ impl CoreApp for App {
         rt_app: Rc<dyn RtApp>,
         rt_surface: Rc<dyn RtSurface>,
     ) -> Result<Rc<dyn CoreSurface>, CoreError> {
-        let surface = Surface::new(
-            id,
-            Rc::clone(&self.state),
-            Arc::new(config),
-            Rc::clone(&rt_app),
-            rt_surface,
-        )?;
-        self.add_surface(id, &surface, &rt_app);
-        Ok(surface)
+        Ok(self.create_surface(id, config, rt_app, rt_surface)?)
     }
 
     fn delete_surface(&self, id: SurfaceId) {