speculative: copy_kv_position primitive for tree drafting KV remap

SGLang-style "write-all, copy-move on acceptance" approach: after tree
verification, physically copy an accepted sibling's K/V from its
physical cache slot to the canonical sequential position.

New CUDA kernel: copy_kv_position_kernel in reshape_and_cache.cu.
For one token (src_pos → dst_pos), copies head_dim × num_kv_heads BF16
elements in both K and V pools. Grid = num_kv_heads, block = head_dim.
Cost for one token across 36 layers: ~5.3 MB D2D copy @ 900 GB/s = <6μs.

Rust FFI: copy_kv_position(k_pool, v_pool, block_ids, src_pos, dst_pos,
num_kv_heads, head_dim, block_size, stream).

PagedKVCache method: copy_kv_position(slot, src_pos, dst_pos) — uploads
block_ids for the sequence, calls the kernel per layer. This is the
primitive needed by tree drafting: when a non-primary sibling at cache
position P+2 is accepted as the "true" token for target position P+1,
call copy_kv_position(slot, P+2, P+1) then truncate to P+2.

Next: wire into bench-eagle3 tree drafting loop with top-2 siblings.

crates/xserv-model/src/paged_kv_cache.rs

@@ -515,6 +515,51 @@ impl PagedKVCache {
         Ok(())
     }
 
+    /// Copy K/V data from `src_pos` to `dst_pos` within the same slot, across
+    /// all layers. Used by tree speculative decoding to remap an accepted
+    /// sibling's K/V to the canonical sequential position after acceptance.
+    ///
+    /// Requires: both positions within the currently-allocated block range.
+    pub fn copy_kv_position(&self, slot: usize, src_pos: usize, dst_pos: usize) {
+        let state = self.seq_states[slot]
+            .as_ref()
+            .expect("copy_kv_position: slot not registered");
+        assert!(
+            src_pos < state.seq_len && dst_pos < state.seq_len,
+            "copy_kv_position: positions must be within seq_len"
+        );
+        // Upload this sequence's block_ids to a small GPU buffer.
+        let block_ids_host: Vec<i32> = state.block_ids.iter().map(|&b| b as i32).collect();
+        let bytes: &[u8] = unsafe {
+            std::slice::from_raw_parts(
+                block_ids_host.as_ptr() as *const u8,
+                block_ids_host.len() * 4,
+            )
+        };
+        let mut ids_buf =
+            xserv_cuda::allocator::cached_alloc(bytes.len()).expect("alloc block_ids for copy");
+        ids_buf.copy_from_host(bytes).unwrap();
+        let ids_ptr = ids_buf.as_ptr() as *const i32;
+
+        let stream = xserv_cuda::current_stream_raw();
+        let num_layers = self.k_pools.len();
+        for layer in 0..num_layers {
+            unsafe {
+                xserv_kernels::copy_kv_position(
+                    self.k_pools[layer].as_ptr() as *mut std::ffi::c_void,
+                    self.v_pools[layer].as_ptr() as *mut std::ffi::c_void,
+                    ids_ptr,
+                    src_pos,
+                    dst_pos,
+                    self.num_kv_heads,
+                    self.head_dim,
+                    BLOCK_SIZE,
+                    stream,
+                );
+            }
+        }
+    }
+
     /// Refresh the host-side block table + context lens from `seq_states`,
     /// then upload to GPU. Call once per decode step before the paged kernel.
     pub fn sync_to_gpu(&mut self) {