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-kernels/src/embedding.rs

@@ -35,19 +35,32 @@ pub fn embedding(table: &Tensor, token_ids: &[u32]) -> Tensor {
         assert!((tid as usize) < vocab_size, "token_id {tid} out of bounds (vocab_size={vocab_size})");
     }
 
+    embedding_device_ids(table, ids_gpu.as_ptr() as *const c_void, num_tokens)
+}
+
+/// Embedding lookup with token ids already on the GPU (u32, [num_tokens]).
+/// Used by the CUDA-graph decode path, where ids live in a persistent device
+/// buffer updated outside the captured region (no bounds check possible here).
+pub fn embedding_device_ids(table: &Tensor, ids_gpu: *const c_void, num_tokens: usize) -> Tensor {
+    assert_eq!(table.ndim(), 2);
+    assert!(table.is_contiguous());
+    assert!(matches!(table.device(), Device::Cuda(_)));
+    let hidden_size = table.shape()[1];
+    let vocab_size = table.shape()[0];
+
     let out = Tensor::empty(&[num_tokens, hidden_size], table.dtype(), table.device());
 
     unsafe {
         match table.dtype() {
             DType::F32 => launch_embedding_f32(
-                table.data_ptr() as _, ids_gpu.as_ptr() as _,
+                table.data_ptr() as _, ids_gpu,
                 out.data_ptr() as *mut c_void,
-                num_tokens as i32, hidden_size as i32, vocab_size as i32, std::ptr::null_mut(),
+                num_tokens as i32, hidden_size as i32, vocab_size as i32, xserv_cuda::current_stream_raw(),
             ),
             DType::BF16 => launch_embedding_bf16(
-                table.data_ptr() as _, ids_gpu.as_ptr() as _,
+                table.data_ptr() as _, ids_gpu,
                 out.data_ptr() as *mut c_void,
-                num_tokens as i32, hidden_size as i32, vocab_size as i32, std::ptr::null_mut(),
+                num_tokens as i32, hidden_size as i32, vocab_size as i32, xserv_cuda::current_stream_raw(),
             ),
             _ => panic!("unsupported dtype for embedding"),
         }