fix(neuron,candle): name lifetime on acquire_pool_lock

Lifetime elision fails when a function has two reference parameters
and returns a borrow: rustc can't infer whether the MutexGuard's
lifetime ties to `pool` or `model_id`. The non-CUDA build skipped
this code path (cfg-gated), so the error only surfaced on the GPU
build at https://git.lair.cafe/helexa/cortex/actions/runs/162.

The guard borrows the pool, so name the lifetime on `pool` and the
return type. `model_id` keeps its independent (elided) lifetime.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/neuron/src/harness/candle.rs

@@ -453,10 +453,10 @@ const POOL_LOCK_WARN_THRESHOLD: Duration = Duration::from_secs(2);
 /// the warn happens at the call site — the request whose lock-wait is
 /// slow is the one that knows its prompt_len and other context.
 #[cfg(feature = "cuda")]
-async fn acquire_pool_lock(
+async fn acquire_pool_lock<'a>(
-    pool: &tokio::sync::Mutex<super::tp::WorkerPool>,
+    pool: &'a tokio::sync::Mutex<super::tp::WorkerPool>,
     model_id: &str,
-) -> tokio::sync::MutexGuard<'_, super::tp::WorkerPool> {
+) -> tokio::sync::MutexGuard<'a, super::tp::WorkerPool> {
     let start = std::time::Instant::now();
     // Tick once at the threshold so a stuck request shows up in
     // journalctl even while it's still waiting. Without this the wait