feat(neuron): TP step watchdog aborts wedged collectives (#17 Stage 2)

Make a hung NCCL collective recoverable instead of a permanent brick.
Today a wedged collective hangs the in-process leader thread forever, and
even Stage 1's recovery can't help — its unload's DropTp queues behind the
stuck thread and hangs too.

- Cache the leader's NCCL Comm handle async-side at init (new cuda-gated
  Job::GetLeaderComm → DeviceWorkerHandle::get_leader_comm → stored on
  WorkerPool.leader_comm). Fetched while the thread is responsive — a
  wedged thread can't service the fetch, which is why it's cached up front.
- Wrap the leader forward in both generate_step and
  generate_step_with_images in tokio::time::timeout (default 120s,
  NEURON_TP_STEP_TIMEOUT_S). On expiry the watchdog calls
  Comm::abort() (ncclCommAbort) on the cached handle from the async
  thread — the one NCCL op sanctioned concurrently with an in-flight
  collective — which unblocks the leader thread, then fails the step
  WITHOUT draining (workers are wedged too; recovery's unload kills them).
  The error is a device fault → poison → Stage 1 auto-recovery, which now
  completes because the leader thread is responsive again.
- Bumps the cudarc patch to dbc425a (adds the Drop-must-not-panic fix so
  the post-abort comm teardown during recovery doesn't double-abort-panic).

Logs the whole sequence at ERROR with greppable `tp watchdog:` /
`ncclCommAbort` markers so a real-world hang leaves a forensic trail —
verification is by inspecting journals after real hangs, not a synthetic
harness. cuda-gated → validated by the blackwell build.

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

Cargo.lock

@@ -905,7 +905,7 @@ dependencies = [
 [[package]]
 name = "cudarc"
 version = "0.19.7"
-source = "git+https://github.com/grenade/cudarc?rev=4dff0be72d8a685d6691a6a53d4c95e1fe932277#4dff0be72d8a685d6691a6a53d4c95e1fe932277"
+source = "git+https://github.com/grenade/cudarc?rev=dbc425aa865c178f38a3ec838f1f7a4da3146358#dbc425aa865c178f38a3ec838f1f7a4da3146358"
 dependencies = [
  "float8",
  "half",