Use floor division for the spec's 25/24 RBF feerate rule

The spec says the 25/24 multiplicative feerate is "rounded down", but
min_rbf_feerate used ceiling division. This made the computed minimum 1
sat/kwu too high when prev * 25 is not evenly divisible by 24, which
could reject valid counterparty feerates.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jkczyz

lightning/src/ln/channel.rs

@@ -6777,7 +6777,7 @@ fn get_v2_channel_reserve_satoshis(
 /// multiplicative 25/24 rule alone would be insufficient.
 fn min_rbf_feerate(prev_feerate: u32) -> FeeRate {
 	let flat_increment = (prev_feerate as u64).saturating_add(25);
-	let spec_increment = ((prev_feerate as u64) * 25).div_ceil(24);
+	let spec_increment = (prev_feerate as u64) * 25 / 24;
 	FeeRate::from_sat_per_kwu(cmp::max(flat_increment, spec_increment))
 }
 

lightning/src/ln/splicing_tests.rs

@@ -4674,7 +4674,7 @@ fn test_splice_rbf_insufficient_feerate_high() {
 	expect_splice_pending_event(&nodes[0], &node_id_1);
 	expect_splice_pending_event(&nodes[1], &node_id_0);
 
-	// prev=1000: flat increment gives 1000+25=1025, 25/24 rule gives ceil(1000*25/24)=1042.
+	// prev=1000: flat increment gives 1000+25=1025, 25/24 rule gives 1000*25/24=1041.
 	// Feerate 1025 satisfies the flat increment but not 25/24 — rejected.
 	reenter_quiescence(&nodes[0], &nodes[1], &channel_id);
 
@@ -4689,13 +4689,13 @@ fn test_splice_rbf_insufficient_feerate_high() {
 	let tx_abort = get_event_msg!(nodes[1], MessageSendEvent::SendTxAbort, node_id_0);
 	assert_eq!(tx_abort.channel_id, channel_id);
 
-	// Feerate 1042 satisfies both rules — accepted.
+	// Feerate 1041 satisfies both rules — accepted.
 	nodes[0].node.handle_tx_abort(node_id_1, &tx_abort);
 
 	let tx_init_rbf = msgs::TxInitRbf {
 		channel_id,
 		locktime: 0,
-		feerate_sat_per_1000_weight: 1042,
+		feerate_sat_per_1000_weight: 1041,
 		funding_output_contribution: Some(added_value.to_sat() as i64),
 	};