build(neuron): patch cudarc to expose Comm::abort/get_async_error (#17 Stage 2)

#17 Stage 2 (TP hang-recovery) needs to call ncclCommAbort on a LIVE
communicator from another thread — to unblock a collective wedged on a
dead/hung peer so the ranks can resync. No cudarc release (incl. main)
exposes this: the safe Comm only aborts in Drop, which can't fire while a
stuck thread holds an Arc<Comm> clone.

Pin neuron's cudarc 0.19.7 to a fork (grenade/cudarc @ nccl-comm-abort,
rev 4dff0be) adding three thin methods — Comm::abort, get_async_error,
and a raw comm() accessor — to be submitted upstream. The patch targets
0.19.x only; candle's transitive cudarc 0.17.8 stays on crates.io.

Foundation only; the watchdog + abort + comm-rebuild that consume these
land in follow-up commits (cuda-gated → validated by the blackwell build).

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

Cargo.lock

@@ -905,8 +905,7 @@ dependencies = [
 [[package]]
 name = "cudarc"
 version = "0.19.7"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1cea5f10a99e025c1b44ae2354c2d8326b25ddbd0baf76bde8e55cfd4018a2cc"
+source = "git+https://github.com/grenade/cudarc?rev=4dff0be72d8a685d6691a6a53d4c95e1fe932277#4dff0be72d8a685d6691a6a53d4c95e1fe932277"
 dependencies = [
  "float8",
  "half",