Clean up a stale Vulkan-layer registration at host startup

register_vk_layer writes an HKCU implicit-layer entry that makes the loader pull
our DLL into every Vulkan app; it's session-scoped (unregistered on clean exit).
If the host crashed or was killed while registered, the entry leaked and kept
loading our DLL into every Vulkan process until the next clean run.

Add cleanup_stale_vk_layer(), called once at the top of run(): since registration
is opt-in per session, anything registered at startup is a crash leftover, so it
removes it (delegates to unregister_vk_layer). No-op when nothing is registered.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

host/src/vk_layer_setup.hpp

@@ -21,4 +21,10 @@ bool register_vk_layer(const std::wstring& target_image);
 // on host shutdown as a belt-and-suspenders cleanup).
 void unregister_vk_layer();
 
+// Call once at host startup. The layer is session-scoped (registered only while the tool runs,
+// unregistered on clean exit), so any registration present at startup is a leftover from a host that
+// crashed/was killed while registered -- which would otherwise keep loading our DLL into EVERY Vulkan
+// app until the next clean run. This removes that stale registration. No-op if nothing is registered.
+void cleanup_stale_vk_layer();
+
 } // namespace coop