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/cocoa.ll

@@ -1670,3 +1670,16 @@ go:
 out:
   ret void
 }
+
+; ---- the native window, for a graphics API that makes its own surface ------------------
+; win32.ll's contract: on macOS the view the game draws in (a Metal-backed layer would hang
+; off it); there is no module instance to hand over, so that one is null.
+define ptr @win_native_window() {
+entry:
+  %v = load ptr, ptr @W_view
+  ret ptr %v
+}
+define ptr @win_native_instance() {
+entry:
+  ret ptr null
+}

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
+}

runtime/native/gl_win_nowin.ll

@@ -42,3 +42,13 @@ define void @win_toggle_fullscreen() {
 entry:
   ret void
 }
+
+; headless: no window, so nothing for a graphics API to make a surface on (win32.ll has the real ones)
+define ptr @win_native_window() {
+entry:
+  ret ptr null
+}
+define ptr @win_native_instance() {
+entry:
+  ret ptr null
+}

runtime/native/win32.ll

@@ -255,6 +255,21 @@ out:
   ret void
 }
 
+; ---- the native window, for a graphics API that makes its own surface ------------------
+; A Vulkan swapchain is created on the window itself (VkWin32SurfaceCreateInfoKHR takes the
+; module instance and the HWND), not through a context this file attaches. Null before
+; win_open has made the window; the same names are stubs returning null in a headless build.
+define ptr @win_native_window() {
+entry:
+  %h = load ptr, ptr @W_hwnd
+  ret ptr %h
+}
+define ptr @win_native_instance() {
+entry:
+  %inst = call ptr @GetModuleHandleA(ptr null)
+  ret ptr %inst
+}
+
 ; set (%on != 0) or clear a key's bit in the 256-bit held set
 define void @win_held_bit(i32 %k, i32 %on) {
 entry: