Add Weight::checked_mul_by_fee_rate method

Mirror FeeRate::checked_mul_by_weight functionality by adding a symmetrical method to Weight.
This allows users to calculate fees starting from either Weight or FeeRate with consistent behavior.

units/src/fee.rs

@@ -191,6 +191,21 @@ impl ops::Div<FeeRate> for Amount {
     fn div(self, rhs: FeeRate) -> Self::Output { self.checked_div_by_fee_rate_floor(rhs).unwrap() }
 }
 
+impl Weight {
+    /// Checked fee rate multiplication.
+    ///
+    /// Computes the absolute fee amount for a given [`FeeRate`] at this weight.
+    /// When the resulting fee is a non-integer amount, the amount is rounded up,
+    /// ensuring that the transaction fee is enough instead of falling short if
+    /// rounded down.
+    ///
+    /// [`None`] is returned if an overflow occurred.
+    #[must_use]
+    pub const fn checked_mul_by_fee_rate(self, fee_rate: FeeRate) -> Option<Amount> {
+        fee_rate.checked_mul_by_weight(self)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;