feat(runtime): the native window, for a graphics API that makes its own surface

win_native_window / win_native_instance hand a Vulkan swapchain what it is created on: the
HWND and module instance on Windows (VkWin32SurfaceCreateInfoKHR), the game's view on macOS.
Headless builds define both as null (weak on macOS, so a windowed link keeps cocoa.ll's), so
code that asks for them links everywhere and simply finds no window.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

runtime/native/gl.ll

@@ -366,3 +366,13 @@ entry:
   %t = add i64 %m, %us
   ret i64 %t
 }
+
+; headless on macOS: no window, so nothing for a graphics API to make a surface on
+define weak ptr @win_native_window() {
+entry:
+  ret ptr null
+}
+define weak ptr @win_native_instance() {
+entry:
+  ret ptr null
+}