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/gemm/naive.cu

@@ -1,4 +1,5 @@
 #include <cuda_bf16.h>
+#include "../common.cuh"
 
 // Naive GEMM: each thread computes one element of C.
 // C[i][j] = sum_k A[i][k] * B[k][j]
@@ -46,6 +47,7 @@ void launch_gemm_naive_bf16(
     gemm_naive_bf16<<<grid, block, 0, (cudaStream_t)stream>>>(
         (const __nv_bfloat16*)A, (const __nv_bfloat16*)B, (__nv_bfloat16*)C, M, N, K
     );
+    CUDA_CHECK_LAST_ERROR();
 }
 
 void launch_gemm_naive_f32(
@@ -57,6 +59,7 @@ void launch_gemm_naive_f32(
     gemm_naive_f32<<<grid, block, 0, (cudaStream_t)stream>>>(
         (const float*)A, (const float*)B, (float*)C, M, N, K
     );
+    CUDA_CHECK_LAST_ERROR();
 }
 
 } // extern "C"