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/normalization/rmsnorm.cu

@@ -111,6 +111,7 @@ void launch_rmsnorm_f32(const void* x, const void* gamma, void* out,
     if (block < 32) block = 32;
     rmsnorm_f32<<<rows, block, 0, (cudaStream_t)stream>>>(
         (const float*)x, (const float*)gamma, (float*)out, hidden_size, eps);
+    CUDA_CHECK_LAST_ERROR();
 }
 
 void launch_rmsnorm_bf16(const void* x, const void* gamma, void* out,
@@ -120,6 +121,7 @@ void launch_rmsnorm_bf16(const void* x, const void* gamma, void* out,
     rmsnorm_bf16<<<rows, block, 0, (cudaStream_t)stream>>>(
         (const __nv_bfloat16*)x, (const __nv_bfloat16*)gamma,
         (__nv_bfloat16*)out, hidden_size, eps);
+    CUDA_CHECK_LAST_ERROR();
 }
 
 void launch_add_rmsnorm_bf16(const void* x, const void* residual, const void* gamma,
@@ -132,6 +134,7 @@ void launch_add_rmsnorm_bf16(const void* x, const void* residual, const void* ga
         (const __nv_bfloat16*)gamma,
         (__nv_bfloat16*)normed_out, (__nv_bfloat16*)sum_out,
         hidden_size, eps);
+    CUDA_CHECK_LAST_ERROR();
 }
 
 }