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.toml

@@ -61,3 +61,12 @@ eventsource-stream = "0.2"
 # workspace crates
 cortex-core = { path = "crates/cortex-core" }
 cortex-gateway = { path = "crates/cortex-gateway" }
+
+# Patched cudarc (affects neuron's 0.19.x only; candle's 0.17.x is
+# untouched since the fork is 0.19.7 and doesn't satisfy a 0.17 req). Adds
+# Comm::abort / get_async_error / raw comm() — needed for #17 Stage 2 TP
+# hang-recovery (abort a wedged collective from another thread, then
+# rebuild the comm). Pinned to a fork revision pending upstream review
+# (grenade/cudarc @ nccl-comm-abort).
+[patch.crates-io]
+cudarc = { git = "https://github.com/grenade/cudarc", rev = "4dff0be72d8a685d6691a6a53d4c95e1fe932277" }