feat: add page-aligned index validation for CP HiCache direct transfers

Add validate_page_aligned_token_indices utility and apply it across
HiCache write/load paths, NSA indexer transfers, and CUDA direct copy
kernels to reject malformed (partial, misaligned, non-contiguous) page
groups before they reach native transfer code. Also validate supported
CP HiCache backend/layout combinations at server startup.

sgl-kernel/csrc/kvcacheio/transfer.cu

@@ -686,6 +686,38 @@ inline void transfer_page_direct(
           /* non_blocking= */ true);
 }
 
+inline void validate_page_first_direct_indices(
+    const int64_t* indices_ptr, int64_t num_indices, int64_t page_size, const char* name) {
+  if (num_indices == 0) {
+    return;
+  }
+  for (int64_t i = 0; i < num_indices; i += page_size) {
+    const int64_t start = indices_ptr[i];
+    TORCH_CHECK(
+        start % page_size == 0,
+        name,
+        " page groups must start at page boundaries: group=",
+        i / page_size,
+        " start=",
+        start,
+        " page_size=",
+        page_size);
+    for (int64_t offset = 1; offset < page_size; ++offset) {
+      TORCH_CHECK(
+          indices_ptr[i + offset] == start + offset,
+          name,
+          " page groups must be contiguous page spans: group=",
+          i / page_size,
+          " offset=",
+          offset,
+          " expected=",
+          start + offset,
+          " got=",
+          indices_ptr[i + offset]);
+    }
+  }
+}
+
 void transfer_kv_direct(
     const std::vector<at::Tensor>& src_layers,
     std::vector<at::Tensor> dst_layers,
@@ -749,6 +781,8 @@ inline void transfer_kv_page_first_direct_impl(
   const int64_t num_pages = src_indices_cpu.size(0) / page_size;
   int64_t* src_indices_ptr = src_indices_cpu.data_ptr<int64_t>();
   int64_t* dst_indices_ptr = dst_indices_cpu.data_ptr<int64_t>();
+  validate_page_first_direct_indices(src_indices_ptr, src_indices_cpu.numel(), page_size, "src_indices");
+  validate_page_first_direct_indices(dst_indices_ptr, dst_indices_cpu.numel(), page_size, "dst_indices");
 
   auto fallback_to_page_copy = [&]() {
     if constexpr (IsLf2Pf) {