fix: CoinSelector::implied_fee

The logic was wrong before because it was making the RBF constraints
take precedence. Instead, we should be taking the maximum between
the fee calculated from the target feerate and the minimum fee that
satisfies the RBF constraints.

src/coin_selector.rs

@@ -229,13 +229,18 @@ impl<'a> CoinSelector<'a> {
 
     /// The fee the current selection and `drain_weight` should pay to satisfy `target_fee`.
     ///
-    /// `drain_weight` can be 0 to indicate no draining output
+    /// This compares the fee calculated from the target feerate with the fee calculated from the
+    /// [`Replace`] constraints and returns the larger of the two.
+    ///
+    /// `drain_weight` can be 0 to indicate no draining output.
     pub fn implied_fee(&self, target: Target, drain_weights: DrainWeights) -> u64 {
         let mut implied_fee = self.implied_fee_from_feerate(target, drain_weights);
 
         if let Some(replace) = target.fee.replace {
-            implied_fee =
-                replace.min_fee_to_do_replacement(self.weight(target.outputs, drain_weights));
+            implied_fee = Ord::max(
+                implied_fee,
+                replace.min_fee_to_do_replacement(self.weight(target.outputs, drain_weights)),
+            );
         }
 
         implied_fee