feat: port Vulkan headers and ICD loader; wire into kwin

KWin does an unconditional find_package(Vulkan REQUIRED), and CMake's
FindVulkan needs BOTH Vulkan_INCLUDE_DIR and Vulkan_LIBRARY. Mesa already
builds the Vulkan drivers (libvulkan_intel/_radeon/_lvp) but an ICD is not
the API surface a consumer links against, so kwin could not configure.

vulkan-headers: header-only, but VulkanHeadersConfig.cmake is GENERATED at
install time rather than shipped in the tarball, and Vulkan-Loader
hard-requires it -- a hand copy of include/ satisfies FindVulkan and then
fails the loader. Run upstream's cmake install instead.

vulkan-loader: one real port. vk_loader_platform.h gates the whole POSIX
arm (DIRECTORY_SYMBOL, the dlopen wrappers, manifest search) on an OS list
that predates Redox, so the build collapsed with 'DIRECTORY_SYMBOL'
undeclared. Redox provides dlopen/dlsym and POSIX paths via relibc, so it
belongs in the same arm as Linux; that header is the only file under
loader/ doing OS detection. WSI backends are left off: Red Bear is
Wayland-only and the compositor talks to Mesa's ICDs directly.

Both shadow the untested recipes/wip/libs/graphics/{vulkan-headers,
libvulkan} via the local-over-WIP mechanism, per the WIP ownership rule
that the shipping version lives in local/recipes/.

Verified: kwin now reports
  -- Found Vulkan: .../sysroot/lib/libvulkan.so (found version 1.4.321)

kwin also gains its missing kf6-knewstuff dep -- its
find_package(KF6 COMPONENTS NewStuff) is REQUIRED, so without the dep the
framework was never staged into kwin's sysroot.

local/recipes/libs/vulkan-headers/source/.github/pull_request_template.md

@@ -0,0 +1,18 @@
+<!-- Please note when contributing what files this repository actually is responsible for.
+
+Vulkan-Headers exists as a publishing mechanism for headers and related material sourced from multiple other repositories. If you have a problem with that material, it should *not* be reported here, but in the appropriate repository:
+
+This repository is responsible for the following files
+
+* BUILD.gn
+* BUILD.md
+* CMakeLists.txt
+* tests/*
+* CODE_OF_CONDUCT.md
+* LICENSE.txt
+* README.md
+* Non-API headers
+  * include/vulkan/vk_icd.h
+  * include/vulkan/vk_layer.h
+
+-->