build(neuron): bump cudarc fork to 63327a2 (idempotent abort + Comm Send+Sync)

The fork's new commit makes `Comm: Send + Sync` (asserting NCCL's
thread-safety invariant upstream) and makes `Comm::abort` idempotent via
an `aborted` flag (so abort-then-Drop can't double-free) — strictly
better than the previous Drop-no-panic workaround, and the `abort()`
signature is unchanged so the watchdog call site is unaffected.

Because `Comm` is now `Send + Sync`, `Arc<Comm>` and the `SendComm` /
`NcclState` wrappers auto-derive `Send`/`Sync`, which conflicts (E0119)
with neuron's manual `unsafe impl`s. Remove the four now-redundant impls
— the safety assertion lives upstream in cudarc where it belongs. The
conflict is in cuda-gated code, so only the CUDA type-check catches it
(non-cuda build + clippy + tests stay green).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/neuron/src/harness/tp/nccl_state.rs

@@ -119,40 +119,25 @@ mod cuda_impl {
         }
     }
 
-    /// `Arc<Comm>` doesn't impl `Send` because `Comm` wraps a raw
-    /// `ncclComm_t` pointer. The NCCL contract is "operations against a
-    /// given comm must be serialised", not "the handle must stay on the
-    /// thread that created it" — so it's safe to move an `Arc<Comm>`
-    /// across threads as long as no concurrent ops are issued. The
-    /// pool's outer Mutex serialises us into `spawn_blocking`, so this
-    /// wrapper at the move boundary is the only thing missing.
+    /// Thin newtype over `Arc<Comm>`, kept for call-site clarity — it marks
+    /// the points where a comm handle is intentionally moved across threads
+    /// (e.g. cached async-side for the TP step watchdog's `ncclCommAbort`).
     ///
-    /// `Sync` is also marked safe because the `Arc<Comm>` clones held
-    /// by the row-parallel layers are only used from the
-    /// `spawn_blocking` thread driving the forward pass; concurrent
-    /// access from another thread would still be a bug.
+    /// `Send`/`Sync` are provided upstream by `cudarc`'s `Comm` (which
+    /// asserts the NCCL thread-safety invariant, including aborting from a
+    /// different thread than one inside a collective), so this type derives
+    /// them automatically — no manual `unsafe impl` here.
     pub struct SendComm(pub Arc<Comm>);
 
-    // SAFETY: see the doc-comment above; the invariant is enforced at
-    // the call site (pool Mutex + single spawn_blocking thread), not at
-    // the type level.
-    unsafe impl Send for SendComm {}
-    unsafe impl Sync for SendComm {}
-
     impl SendComm {
         pub fn into_inner(self) -> Arc<Comm> {
             self.0
         }
     }
 
-    // SAFETY: `cudarc::nccl::Comm` contains a raw `ncclComm_t` pointer
-    // (libnccl-allocated state). NCCL requires that operations against
-    // one Comm be issued one at a time; we serialise access by storing
-    // NcclState behind a Mutex in `WorkerPool`. The Comm itself is
-    // move-safe — NCCL doesn't track the calling OS thread, only the
-    // stream the operations are dispatched against.
-    unsafe impl Send for NcclState {}
-    unsafe impl Sync for NcclState {}
+    // `NcclState`'s `Send`/`Sync` are auto-derived: its `Arc<Comm>` and
+    // `Arc<CudaContext>` fields are now `Send`/`Sync` (cudarc asserts the
+    // comm thread-safety invariant), so no manual `unsafe impl` is needed.
 
     /// Generate a fresh NCCL `Id` and return it hex-encoded. Used by
     /// the leader to mint the shared communicator id which is then