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/win32.ll

@@ -547,6 +547,16 @@ entry:
   %inst = call ptr @GetModuleHandleA(ptr null)
   ret ptr %inst
 }
+; macOS only (cocoa.ll): the CAMetalLayer MoltenVK presents to
+define ptr @win_metal_layer() {
+entry:
+  ret ptr null
+}
+; the window is always drawn to here (cocoa.ll asks macOS whether it is covered)
+define i32 @win_visible() {
+entry:
+  ret i32 1
+}
 
 ; set (%on != 0) or clear a key's bit in the 256-bit held set
 define void @win_held_bit(i32 %k, i32 %on) {