opt: Tie-break UTXO sort by waste for BnB

Since we are searching for the minimal waste, we sort UTXOs with equal
effective value by ascending waste to be able to cut barren branches
earlier.

Author: murchandamus

src/wallet/coinselection.cpp

@@ -25,10 +25,14 @@ static util::Result<SelectionResult> ErrorMaxWeightExceeded()
                          "Please try sending a smaller amount or manually consolidating your wallet's UTXOs")};
 }
 
-// Descending order comparator
+// Sort by descending (effective) value prefer lower waste on tie
 struct {
     bool operator()(const OutputGroup& a, const OutputGroup& b) const
     {
+        if (a.GetSelectionAmount() == b.GetSelectionAmount()) {
+            // Lower waste is better when effective_values are tied
+            return (a.fee - a.long_term_fee) < (b.fee - b.long_term_fee);
+        }
         return a.GetSelectionAmount() > b.GetSelectionAmount();
     }
 } descending;