cuda: infrastructure for whole-step CUDA graph capture

- Thread-local launch stream (xserv_cuda::stream): every kernel
  wrapper, cublasSetStream, and NCCL collective now launches on
  current_stream_raw() — the legacy null stream by default (behavior
  unchanged), or the capture stream installed via push_stream during
  graph capture. Capture is impossible on the legacy stream.
- Allocator retain mode: blocks freed inside a retain window are
  quarantined (RetainedBlocks) instead of pooled, so an instantiated
  graph keeps exclusive ownership of every intermediate buffer it
  references across replays.
- Capture mode GLOBAL -> THREAD_LOCAL: concurrent TP rank threads
  must not poison each other's captures with their own cudaMallocs.
- embedding_device_ids / rope_inplace_device_pos: variants reading
  token ids / positions from persistent device buffers, replacing the
  per-call host upload that a captured region cannot contain.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/xserv-model/src/paged_kv_cache.rs

@@ -349,7 +349,7 @@ impl PagedKVCache {
                 k_pool_ptr, v_pool_ptr,
                 block_ids_gpu.as_ptr() as *const i32,
                 num_tokens, nkv, hd, start_pos, bs,
-                std::ptr::null_mut(),
+                xserv_cuda::current_stream_raw(),
             );
         }
         // block_ids_gpu drops here; the launch on the null stream will have
@@ -397,7 +397,7 @@ impl PagedKVCache {
                 k_pool_ptr, v_pool_ptr,
                 bt_ptr, cl_ptr,
                 batch, nkv, hd, BLOCK_SIZE, self.max_blocks_per_seq,
-                std::ptr::null_mut(),
+                xserv_cuda::current_stream_raw(),
             );
         }
     }