kernels/cuda: paged-attention kernel, dispatch, pinned host memory

CUDA layer for the paged-KV + swap work:
- csrc: new paged_attention.cu plus updates across attention/gemm/norm/
  activation/embedding/reduce kernels and common.cuh.
- xserv-kernels: new dispatch module and kernel-binding updates.
- xserv-cuda: cudaMallocHost/FreeHost bindings + PinnedBuffer (host swap
  pool backing) and offset-aware D2H/H2D copies used to move KV blocks
  between the GPU pool and pinned host memory.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/xserv-kernels/src/lib.rs

@@ -1,5 +1,6 @@
 pub mod activation;
 pub mod attention;
+pub mod dispatch;
 pub mod embedding;
 pub mod gemm;
 pub mod layernorm;
@@ -10,7 +11,7 @@ pub mod transpose;
 
 pub use activation::{add, gelu, mul, scale, silu, silu_mul};
 pub use transpose::{merge_heads_gpu, repeat_kv_gpu, reshape_heads_gpu, strided_to_contiguous_gpu, transpose_for_rope_gpu, transpose_from_rope_gpu};
-pub use attention::{attention, decode_attention, flash_attention};
+pub use attention::{attention, decode_attention, flash_attention, paged_decode_attention};
 pub use embedding::embedding;
 pub use gemm::{batched_matmul, matmul, GemmBackend};
 pub use layernorm::layernorm;