Deduplicate the funding feerate ceiling in splice_in and splice_out

The 1.5x-of-estimate funding feerate ceiling was open-coded identically in
splice_in and splice_out. Route both through the max_funding_feerate helper so
the ceiling lives in one place.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jkczyz

src/lib.rs

@@ -1578,7 +1578,7 @@ impl Node {
 		{
 			let min_feerate =
 				self.fee_estimator.estimate_fee_rate(ConfirmationTarget::ChannelFunding);
-			let max_feerate = FeeRate::from_sat_per_kwu(min_feerate.to_sat_per_kwu() * 3 / 2);
+			let max_feerate = max_funding_feerate(min_feerate);
 
 			let splice_amount_sats = match splice_amount_sats {
 				FundingAmount::Exact { amount_sats } => amount_sats,
@@ -1767,7 +1767,7 @@ impl Node {
 
 			let min_feerate =
 				self.fee_estimator.estimate_fee_rate(ConfirmationTarget::ChannelFunding);
-			let max_feerate = FeeRate::from_sat_per_kwu(min_feerate.to_sat_per_kwu() * 3 / 2);
+			let max_feerate = max_funding_feerate(min_feerate);
 
 			let funding_template = self
 				.channel_manager