llama : pin mmap-backed CPU weights for faster H2D uploads

Wire the existing GGML_CUDA_REGISTER_HOST path back up: after model load,
cudaHostRegister the mmap pages backing weights kept in system memory.
Recovers pageable-copy losses when MoE experts are streamed to the GPU
during prefill (n-cpu-moe): Qwen3.6-35B-A3B pp2048 1144 -> 1385 t/s on
RTX 3060. Opt-in via GGML_CUDA_REGISTER_HOST=1, unchanged otherwise.

src/llama-mmap.h

@@ -50,11 +50,19 @@ struct llama_mmap {
 
     void unmap_fragment(size_t first, size_t last);
 
+    // pin the pages backing [first, last) with a backend allocator for faster H2D copies,
+    // unpinned in the destructor before the pages are unmapped
+    // returns the number of bytes registered, 0 on failure
+    size_t register_host(size_t first, size_t last, bool (*reg_fn)(void *, size_t), void (*unreg_fn)(void *));
+
     static const bool SUPPORTED;
 
 private:
     struct impl;
     std::unique_ptr<impl> pimpl;
+
+    void * host_reg_addr = nullptr;
+    void (*host_unreg_fn)(void *) = nullptr;
 };
 
 struct llama_mlock {