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>

selfhost/backend/emit_intrin.ludic

@@ -20,6 +20,12 @@ function is_intrinsic(name: pointer) -> bool {
 function emit_bind(rest: pointer) -> pointer { let r = nreg();  emit("  ");  emit(r);  emit(" = ");  emit(rest);  emit("\n");  return r }
 
 function arg_code(e: Node, i: int) -> pointer { let v = emit_expr(e.kids[i]);  return v.code }
+# a buffer handed to the platform layer: a slice's elements, never its header (as an extern's are)
+function arg_buf(e: Node, i: int) -> pointer {
+  let v = emit_expr(e.kids[i])
+  if is_slice_ty(v.ty) { return emit_bind(`load ptr, ptr {slice_field(v.code, 0)}`) }
+  return v.code
+}
 
 function emit_intrinsic(name: pointer, e: Node) -> Val {
   g_intrin_ok = true