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>

csrc/attention/causal_mask.cu

@@ -1,4 +1,5 @@
 #include <cuda_bf16.h>
+#include "../common.cuh"
 
 // Apply causal mask: set scores[row][col] = -inf where col > row + offset.
 // offset is used for KV cache: when query starts at position `offset`,
@@ -39,6 +40,7 @@ void launch_causal_mask_f32(void* scores, int batch, int rows, int cols,
     dim3 grid((cols + block - 1) / block, rows, batch);
     causal_mask_f32<<<grid, block, 0, (cudaStream_t)stream>>>(
         (float*)scores, rows, cols, offset);
+    CUDA_CHECK_LAST_ERROR();
 }
 
 void launch_causal_mask_bf16(void* scores, int batch, int rows, int cols,
@@ -47,6 +49,7 @@ void launch_causal_mask_bf16(void* scores, int batch, int rows, int cols,
     dim3 grid((cols + block - 1) / block, rows, batch);
     causal_mask_bf16<<<grid, block, 0, (cudaStream_t)stream>>>(
         (__nv_bfloat16*)scores, rows, cols, offset);
+    CUDA_CHECK_LAST_ERROR();
 }
 
 }