Unclear way to safely aggregate features in `xrCreateVulkanDeviceKHR`

`xrCreateVulkanDeviceKHR` from `XR_KHR_vulkan_enable2` requires that the implementation aggregate its own extension/feature requirements with the app's requirements passed in `XrVulkanDeviceCreateInfoKHR::vulkanCreateInfo`.

Say, an implementation requires timeline semaphores.
In the case where this feature is not already enabled by the app but the structure enabling it is in the pNext chain (e.g. `VkPhysicalDeviceTimelineSemaphoreFeatures` or `VkPhysicalDeviceVulkan12Features` is present but `timelineSemaphore` is false), since the `vulkanCreateInfo` field is const, the only way to safely enable timeline semaphores is to deep-copy the whole structure chain.

The issue is that the structure chain may contain structures that the implementation is not aware of, and cannot deep-copy.

What is the expected way to resolve this issue?

Read the XR_KHR_vulkan_enable2 definition and the XrVulkanDeviceCreateInfoKHR::vulkanCreateInfo entry, focusing on the required feature and pNext-chain cases described here. Done means the specification documents an unambiguous, safe rule for aggregating implementation and application requirements, including structures the implementation does not know.

Written by the indexing model from the issue text.