feat(render3d): Vulkan in a macOS window through MoltenVK; window built-ins take a slice's elements; the NEAR_FADE SPIR-V

- a CAMetalLayer on the view (cocoa.ll win_metal_layer), VK_EXT_metal_surface, QuartzCore linked
  with Vk.*; the drawable measured after the layer sets the backing scale
- vk_mac.ll opens MoltenVK directly after any loader: a bundle ships only libMoltenVK.dylib
- a covered window is not presented to (win_visible); one frame in flight on macOS
  (R3D_VK_INFLIGHT), with images, buffers and descriptor pools held until it is done
- win_held / win_mouse / win_pad / win_touch / win_text / win_present pass slice elements (arg_buf):
  every windowed program died on its first input poll
- variants.list and SPIR-V for the three NEAR_FADE foliage programs

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

runtime/native/gl.ll

@@ -398,6 +398,15 @@ define weak ptr @win_native_instance() {
 entry:
   ret ptr null
 }
+define weak ptr @win_metal_layer() {
+entry:
+  ret ptr null
+}
+; the window is always drawn to here (cocoa.ll asks macOS whether it is covered)
+define weak i32 @win_visible() {
+entry:
+  ret i32 1
+}
 ; 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.
 ; win_set_title: cocoa.ll's in a windowed build; nothing to retitle headless