refactor(#2500): skip the empty acquire for fully re-entrant nodes

When every resource dep of a node re-enters an ancestor's lock, `owned_reqs`
is empty; the old code still called `acquire(vec![])` and stored a no-op
empty guard in `owned`. Gate the acquire + guard insertion on
`!owned_reqs.is_empty()` — one fewer `borrow_mut` + `HashMap` entry per
fully-re-entrant node in the settle loop. `node_owns` already treats a
missing `owned` entry as non-owning, so behaviour is unchanged.

hive-jobq/src/scheduler.rs

@@ -125,12 +125,17 @@ impl<N, R: Clone + Eq + Hash> Scheduler<N, R> {
                 owned_reqs.push((name, count));
             }
         }
-        // Owned units are all-or-nothing; borrow slots were all confirmed free
-        // above, so this is the only fallible step. Nothing mutated until here.
-        let Some(guard) = self.resources.acquire(owned_reqs) else {
-            return false;
-        };
-        self.owned.entry(id).or_default().push(guard);
+        // Owned units (if any) are all-or-nothing; borrow slots were all
+        // confirmed free above, so acquiring them is the only fallible step.
+        // Nothing is mutated until here. Skip the acquire + guard entirely when
+        // every dep was re-entrant (no owned units): an empty guard would just
+        // be a no-op `Drop` plus a wasted `owned` entry.
+        if !owned_reqs.is_empty() {
+            let Some(guard) = self.resources.acquire(owned_reqs) else {
+                return false;
+            };
+            self.owned.entry(id).or_default().push(guard);
+        }
         for slot in borrows {
             self.borrow_slots.insert(slot, id);
         }