perf: pre-allocate BinaryHeap in astar_bag

Add capacity hint to reduce reallocations in astar_bag BinaryHeap,
similar to the optimization already done for regular astar function.

Co-authored-by: samueltardieu <44656+samueltardieu@users.noreply.github.com>

Author: Copilot

src/directed/astar.rs

@@ -185,7 +185,8 @@ where
     FH: FnMut(&N) -> C,
     FS: FnMut(&N) -> bool,
 {
-    let mut to_see = BinaryHeap::new();
+    // Pre-allocate heap with a reasonable default capacity to reduce reallocations
+    let mut to_see = BinaryHeap::with_capacity(64);
     let mut min_cost = None;
     let mut sinks = HashSet::new();
     to_see.push(SmallestCostHolder {