feat(render3d): Vulkan in the game's window on Windows

- gpu_select takes Vulkan for a window on Windows; gvk_init asks for VK_KHR_surface,
  VK_KHR_win32_surface and VK_KHR_swapchain when the renderer will present. A window elsewhere
  stays on OpenGL with the reason.
- gvk_open opens the window without a GL context and makes the screen images at its client area;
  gvk_swap_make builds the swapchain on the Win32 surface (FIFO with vsync, mailbox or immediate
  without) and rebuilds it when it is out of date, suboptimal or the window changes size.
- gvk_present blits the screen image into the acquired swapchain image, flipped (the screen keeps
  OpenGL's bottom-up rows), and presents it.
- Samplers pointed at a texture unit with u_i and then fed by binding units - the actors do this -
  read that unit's texture; on Vulkan they drew with the white stand-in (the tent, the log, the
  chair, the workbench).
- gl.ll carries a weak win_gl_drawable for headless macOS builds.

On the RTX 3070 Ti the windowed build runs the valley through Vulkan at 3840x2160 with the HUD and
the frame-rate readout (16 fps: every upload still waits on the frame, and buffers are
host-visible). The headless Vulkan frame is unchanged; OpenGL frames byte-identical at the five
viewpoints with 59 self-tests passing. The actor fix is compiled and OpenGL-verified, not yet
seen on the PC; blades at the grass's middle distance still draw black on Vulkan.

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

runtime/native/gl.ll

@@ -376,3 +376,12 @@ define weak ptr @win_native_instance() {
 entry:
   ret ptr null
 }
+; the drawable's size, for a graphics API that follows the window without a GL context (render3d's
+; Vulkan resize check); headless there is no window, so 0 x 0. cocoa.ll's real one wins when linked.
+define weak void @win_gl_drawable(ptr %out) {
+entry:
+  store i32 0, ptr %out
+  %o1 = getelementptr i32, ptr %out, i32 1
+  store i32 0, ptr %o1
+  ret void
+}