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-cuda/src/graph.rs

@@ -50,10 +50,13 @@ impl CudaGraph {
     pub fn begin_capture(&mut self, stream: &CudaStream) -> Result<()> {
         // If we have an old graph, destroy it first
         self.destroy_inner();
+        // THREAD_LOCAL: only "potentially unsafe" CUDA calls (cudaMalloc etc.)
+        // made by THIS thread invalidate the capture. With GLOBAL mode, TP rank
+        // threads capturing concurrently would poison each other's captures.
         error::check(unsafe {
             ffi::cudaStreamBeginCapture(
                 stream.as_raw(),
-                ffi::CUDA_STREAM_CAPTURE_MODE_GLOBAL,
+                ffi::CUDA_STREAM_CAPTURE_MODE_THREAD_LOCAL,
             )
         })
     }