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/gemv.cu

@@ -1,5 +1,6 @@
 #include <cuda_bf16.h>
 #include <cuda_runtime.h>
+#include "../common.cuh"
 
 // Custom GEMV kernel for M=1 decode step (BF16):
 //   y[n] = sum_k x[k] * W[k * N + n]
@@ -88,6 +89,7 @@ void launch_gemv_bf16(
         (float*)y_fp32_buf,
         K, N
     );
+    CUDA_CHECK_LAST_ERROR();
 
     // Convert FP32 -> BF16
     int conv_block = 256;
@@ -97,6 +99,7 @@ void launch_gemv_bf16(
         (__nv_bfloat16*)y_bf16,
         N
     );
+    CUDA_CHECK_LAST_ERROR();
 }
 
 } // extern "C"