Pin BDK 3.1 splice fee accounting

Update splice integration expectations for BDK 3.1's fee rounding fix.

This keeps the remaining LDK/BDK fee-accounting drift explicit.

It prevents future dependency changes from restoring the larger surplus.

Refs #901

Co-Authored-By: HAL 9000

Author: tnull

tests/integration_tests_rust.rs

@@ -1150,15 +1150,13 @@ async fn splice_channel() {
 	expect_channel_ready_event!(node_b, node_a.node_id());
 
 	let expected_splice_in_fee_sat = 251;
-	let expected_splice_in_onchain_cost_sat = 254;
+	let expected_splice_in_onchain_cost_sat = 253;
 
-	// LDK's fee calculation differs from BDK wallet's, which over pays on fees. Rather than giving
-	// the extra fees to the miner, LDK sends it to the channel balance since there may not be a
-	// change output.
-	//
-	// TODO: Some of the discrepancy is addressed upstream, so this number should be adjusted when
-	// updating the BDK wallet dependency. See: https://github.com/bitcoindevkit/bdk_wallet/pull/479
-	let expected_splice_in_lightning_balance_sat = 4_000_003;
+	// BDK 3.1.0 avoids the previous per-UTXO fee rounding during coin selection. Keep the
+	// remaining 2-sat LDK/BDK fee-accounting drift explicit so a dependency change cannot silently
+	// reintroduce the larger surplus. Rather than giving the extra sats to the miner, LDK sends
+	// them to the channel balance since there may not be a change output.
+	let expected_splice_in_lightning_balance_sat = 4_000_002;
 
 	let payments = node_b.list_payments();
 	let payment =