[qtbase] qtbase's `vulkan` feature ships a broken vendored Vulkan loader that breaks app startup

### Is your feature request related to a problem? Please describe. Note: This issue was verified by me and _with help of Claude_ (see claude summary below). This issue was affecting our real world Qt6 app since we switched from prebuild Qt bins to vcpkg. Vcpkg's `qtbase[vulkan]` feature pulls in the full `vulkan` port, which bundles a copy of the Vulkan loader (`libvulkan.so` on Linux, `vulkan-1.dll` on Windows) next to our executable. That loader shouldn't ship with the app at all — the Vulkan loader belongs to the system. On Windows it comes with the GPU driver (that's why the Windows Vulkan SDK ships `vulkan-1.lib` to link against but no `vulkan-1.dll`); on Linux it comes from a distro package like `libvulkan1` (Ubuntu) or `vulkan-icd-loader` (Arch). Qt only needs the Vulkan *headers* to build, because it `dlopen`s the loader at runtime instead of linking it — so `vulkan-headers` is the right dependency, not `vulkan`. On Windows the bundled loader happens to work, so nobody notices. On Linux it doesn't: the vendored loader is built without the WSI surface backends (wayland/xcb/xlib), so it has no working surface support. If it lands on the runtime search path it shadows the real system loader, and the app crashes on startup with `Failed to find vkCreateWaylandSurfaceKHR` — even though `vulkaninfo` shows the system loader supports it fine. Deleting the vendored `.so` fixes it instantly. ### Proposed solution ```jsonc // ports/qtbase/vcpkg.json "vulkan": { "dependencies": [ { "name": "qtbase", "default-features": false, "features": ["gui"] }, - "vulkan" + "vulkan-headers" ] } ``` ### Describe alternatives you've considered Instead of dropping the vendored loader, you could keep depending on `vulkan-loader` and just enable its `xcb`/`xlib`/`wayland` features — that also produces a working `libvulkan.so` with the missing surface functions. This is what we did first. Windows never needed this because its Win32 WSI support isn't gated behind an opt-in feature like Linux's is (see below), so the vendored loader always worked there. Still worse than `vulkan-headers`: you're building and shipping a redundant copy of the loader that duplicates what the OS/GPU driver already installs, with the same shadowing risk if it ever drifts from what the real driver stack expects (new extensions, validation/overlay layers, etc.) — just a *working* duplicate instead of a broken one. ### Additional context **Claude summary — the detailed writeup for the issue body:** ## qtbase's `vulkan` feature ships a broken vendored Vulkan loader that breaks app startup Building `qtbase[vulkan]` silently ships a `libvulkan.so` with no working surface support. If it ends up on the runtime search path (common vcpkg deployment pattern), the app fails to create a Vulkan surface at all — on X11 or Wayland — and never starts. Root cause: Qt never links libvulkan, it `dlopen`s it at runtime via `QLibrary` ([`qbasicvulkanplatforminstance.cpp:46-73`](https://github.com/qt/qtbase/blob/v6.11.0/src/gui/vulkan/qbasicvulkanplatforminstance.cpp#L46-L73)). So depending on the `vulkan` stub port (headers + loader) instead of just `vulkan-headers` builds a loader nobody links against. `volk` already got this exact fix in #25762. Worse than just wasted build time: the vendored loader's WSI backends (xcb/wayland/xlib) are off by default, so if it lands on the runtime search path it silently shadows the real system loader. Hit this for real — got `Failed to find vkCreateWaylandSurfaceKHR` even though `vulkaninfo` showed the system loader supports it fine. Deleting the vendored `.so` fixed it instantly. ### Refs - [volk's vcpkg.json](https://github.com/microsoft/vcpkg/blob/master/ports/volk/vcpkg.json) / [#25762](https://github.com/microsoft/vcpkg/issues/25762) — same fix, already done there - [qxcbvulkaninstance.cpp:16](https://github.com/qt/qtbase/blob/v6.11.0/src/plugins/platforms/xcb/qxcbvulkaninstance.cpp#L16) — xcb plugin's `loadVulkanLibrary` call site - [ports/vulkan-loader/vcpkg.json](https://github.com/microsoft/vcpkg/blob/master/ports/vulkan-loader/vcpkg.json) — WSI features are opt-in, none on by default