Implement RBF fee bumping for unconfirmed transactions

Add `Replace-by-Fee` functionality to allow users to increase fees on
pending outbound transactions, improving confirmation likelihood during
network congestion.

- Uses BDK's `build_fee_bump` for transaction replacement
- Validates transaction eligibility: must be outbound and unconfirmed
- Maintains payment history consistency across wallet updates
- Includes integration tests for various RBF scenarios

Author: Camillarhi

bindings/ldk_node.udl

@@ -277,6 +277,8 @@ interface OnchainPayment {
 	Txid send_to_address([ByRef]Address address, u64 amount_sats, FeeRate? fee_rate);
 	[Throws=NodeError]
 	Txid send_all_to_address([ByRef]Address address, boolean retain_reserve, FeeRate? fee_rate);
+	[Throws=NodeError]
+	Txid bump_fee_rbf(PaymentId payment_id, FeeRate? fee_rate);
 };
 
 interface FeeRate {

src/payment/onchain.rs

@@ -10,6 +10,7 @@
 use std::sync::{Arc, RwLock};
 
 use bitcoin::{Address, Txid};
+use lightning::ln::channelmanager::PaymentId;
 
 use crate::config::Config;
 use crate::error::Error;
@@ -120,4 +121,20 @@ impl OnchainPayment {
 		let fee_rate_opt = maybe_map_fee_rate_opt!(fee_rate);
 		self.wallet.send_to_address(address, send_amount, fee_rate_opt)
 	}
+
+	/// Attempt to bump the fee of an unconfirmed transaction using Replace-by-Fee (RBF).
+	///
+	/// This creates a new transaction that replaces the original one, increasing the fee by the
+	/// specified increment to improve its chances of confirmation.
+	///
+	/// The new transaction will have the same outputs as the original but with a
+	/// higher fee, resulting in faster confirmation potential.
+	///
+	/// Returns the [`Txid`] of the new replacement transaction if successful.
+	pub fn bump_fee_rbf(
+		&self, payment_id: PaymentId, fee_rate: Option<FeeRate>,
+	) -> Result<Txid, Error> {
+		let fee_rate_opt = maybe_map_fee_rate_opt!(fee_rate);
+		self.wallet.bump_fee_rbf(payment_id, fee_rate_opt)
+	}
 }

src/payment/pending_payment_store.rs

@@ -84,10 +84,11 @@ impl StorableObjectUpdate<PendingPaymentDetails> for PendingPaymentDetailsUpdate
 
 impl From<&PendingPaymentDetails> for PendingPaymentDetailsUpdate {
 	fn from(value: &PendingPaymentDetails) -> Self {
-		Self {
-			id: value.id(),
-			payment_update: Some(value.details.to_update()),
-			conflicting_txids: Some(value.conflicting_txids.clone()),
-		}
+		let conflicting_txids = if value.conflicting_txids.is_empty() {
+			None
+		} else {
+			Some(value.conflicting_txids.clone())
+		};
+		Self { id: value.id(), payment_update: Some(value.details.to_update()), conflicting_txids }
 	}
 }

src/payment/store.rs

@@ -291,6 +291,15 @@ impl StorableObject for PaymentDetails {
 			}
 		}
 
+		if let Some(tx_id) = update.txid {
+			match self.kind {
+				PaymentKind::Onchain { ref mut txid, .. } => {
+					update_if_necessary!(*txid, tx_id);
+				},
+				_ => {},
+			}
+		}
+
 		if updated {
 			self.latest_update_timestamp = SystemTime::now()
 				.duration_since(UNIX_EPOCH)
@@ -540,6 +549,7 @@ pub(crate) struct PaymentDetailsUpdate {
 	pub direction: Option<PaymentDirection>,
 	pub status: Option<PaymentStatus>,
 	pub confirmation_status: Option<ConfirmationStatus>,
+	pub txid: Option<Txid>,
 }
 
 impl PaymentDetailsUpdate {
@@ -555,6 +565,7 @@ impl PaymentDetailsUpdate {
 			direction: None,
 			status: None,
 			confirmation_status: None,
+			txid: None,
 		}
 	}
 }
@@ -570,9 +581,9 @@ impl From<&PaymentDetails> for PaymentDetailsUpdate {
 			_ => (None, None, None),
 		};
 
-		let confirmation_status = match value.kind {
-			PaymentKind::Onchain { status, .. } => Some(status),
-			_ => None,
+		let (confirmation_status, txid) = match &value.kind {
+			PaymentKind::Onchain { status, txid, .. } => (Some(*status), Some(*txid)),
+			_ => (None, None),
 		};
 
 		let counterparty_skimmed_fee_msat = match value.kind {
@@ -593,6 +604,7 @@ impl From<&PaymentDetails> for PaymentDetailsUpdate {
 			direction: Some(value.direction),
 			status: Some(value.status),
 			confirmation_status,
+			txid,
 		}
 	}
 }