Export register_host_buffer/unregister via the backend reg so the existing
GGML_CUDA_REGISTER_HOST path in llama-model-loader pins mmap'd expert
weights on Vulkan. Imports host pages through VK_EXT_external_memory_host
into device->pinned_memory, letting the existing pinned fast path in
ggml_vk_buffer_write_2d_async DMA straight from system RAM instead of
bouncing through the staging buffer + blocking host memcpy.
A single Vulkan buffer cannot cover a multi-GB mmap (capped at
device->max_buffer_size), so the region is imported in page-aligned
chunks; a bound check makes tensors straddling a chunk boundary fall back
to staging instead of reading out of bounds. Opt-in via
GGML_CUDA_REGISTER_HOST=1 or GGML_VK_REGISTER_HOST=1, no-op otherwise.
Assisted-by: opencode
Lumpiasty