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>

changes/vulkan-on-macos.md

@@ -0,0 +1,10 @@
+bump: minor
+type: feature
+**render3d's Vulkan renderer runs in a macOS window, through MoltenVK.** `R3D_GFX=vk` (or
+`gpu_request("vulkan")`) now presents to a `CAMetalLayer` on the window (`VK_EXT_metal_surface`,
+the runtime's `win_metal_layer()`), where it used to fall back to OpenGL. The loader also opens
+MoltenVK on its own - `Contents/Frameworks/libMoltenVK.dylib` in a bundle, beside the executable,
+or `$VULKAN_SDK/lib` - so a game needs no Vulkan SDK to ship it. A covered window is not drawn to
+(its layer would hold every frame for a second), and on macOS one frame stays in flight so the CPU
+runs the next tick while the GPU draws. OpenGL stays the default: on an M4 Pro the Vulkan frame is
+still about a third slower, most of it in the alpha-tested foliage.