Importing `Memory::External(ExternalMemorySource::HostAllocation(ptr))`
used to crash with `ERROR_INVALID_EXTERNAL_HANDLE` because:

1. The extension wasn't being enabled on the device — the
   `VkImportMemoryHostPointerInfoEXT` struct chained into
   `vkAllocateMemory` was unknown to the driver.
2. The memory type index was picked by `memory_type_bits.ilog2()`,
   which doesn't account for the host-pointer compatibility set the
   driver advertises via `vkGetMemoryHostPointerPropertiesEXT`.
3. `allocationSize` wasn't rounded up to
   `minImportedHostPointerAlignment`.

Fix all three. When the extension is unsupported the capability stays
off and attempts to import a host allocation now panic with a clear
message pointing at `Capabilities::external_memory_host` rather than
going into `vkAllocateMemory` and getting rejected.

Verified end-to-end with meganeura's `external_buffer` example on
AMD Radeon 780M / RADV PHOENIX (input bytes traverse producer-GPU →
meganeura-GPU with no CPU roundtrip; GPU-vs-CPU max error 2.4e-7).

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>