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/softmax.rs

@@ -14,6 +14,8 @@ pub fn softmax(x: &Tensor) -> Tensor {
 
     let cols = *x.shape().last().unwrap();
     let rows = x.numel() / cols;
+    assert!(rows <= i32::MAX as usize, "too many rows for i32 kernel param");
+    assert!(cols <= i32::MAX as usize, "cols too large for i32 kernel param");
     let out = Tensor::empty(x.shape(), x.dtype(), x.device());
 
     unsafe {