wallet: drop change to fee if amount is dust

This commit refactors the CoinSelector class to properly handle the calculation
of change cost. The change amount is now checked against the cost of change,
and if the change is less than the cost, the cost is added to the transaction fee.
This ensures that the transaction output is correctly adjusted based on the
current fee rate and the size of the change output.

src/wallet/coin-selector.ts

@@ -36,13 +36,25 @@ export class CoinSelector {
 
         const selected = this.selectCoins(pointers, target);
 
-        const change =
+        let change =
             selected.reduce(
                 (acc, index) => acc + pointers[index].effectiveValue,
                 0,
             ) - target;
 
-        // TODO: check if change is lower than dust limit, if so add it to fee
+        // Calculate the cost of change
+        const LONG_TERM_FEERATE = 5 // in sats/vB
+        const outputSize = 31;  // P2WPKH output is 31 B
+        const inputSizeOfChangeUTXO = 68.0; // P2WPKH input is 68.0 vbytes
+
+        const costOfChangeOutput = outputSize * this.feeRate;
+        const costOfSpendingChange = inputSizeOfChangeUTXO * LONG_TERM_FEERATE;
+        const costOfChange = costOfChangeOutput + costOfSpendingChange;
+
+        // Check if change is less than the cost of change
+        if(change <= costOfChange) {
+            change = 0;
+        }
 
         return {
             coins: selected.map((index) => pointers[index].coin),