blinded_path/refactor: make construction generic over forwarding type

diff --git a/lightning/src/blinded_path/payment.rs b/lightning/src/blinded_path/payment.rs
index 03b676a..5766bdf 100644
--- a/lightning/src/blinded_path/payment.rs
+++ b/lightning/src/blinded_path/payment.rs
@@ -161,8 +161,12 @@ impl BlindedPaymentPath {
 		)
 	}
 
-	fn new_inner<ES: EntropySource, T: secp256k1::Signing + secp256k1::Verification>(
-		intermediate_nodes: &[PaymentForwardNode], payee_node_id: PublicKey,
+	fn new_inner<
+		F: ForwardTlvsInfo,
+		ES: EntropySource,
+		T: secp256k1::Signing + secp256k1::Verification,
+	>(
+		intermediate_nodes: &[ForwardNode<F>], payee_node_id: PublicKey,
 		local_node_receive_key: ReceiveAuthKey, dummy_tlvs: &[DummyTlvs], payee_tlvs: ReceiveTlvs,
 		htlc_maximum_msat: u64, min_final_cltv_expiry_delta: u16, entropy_source: ES,
 		secp_ctx: &Secp256k1<T>,
@@ -323,18 +327,42 @@ impl BlindedPaymentPath {
 	}
 }
 
-/// An intermediate node, its outbound channel, and relay parameters.
+mod sealed {
+	pub trait ForwardTlvsInfo {}
+}
+
+/// Common interface for forward TLV types used in blinded payment paths.
+///
+/// Both [`ForwardTlvs`] (channel-based forwarding) and [`TrampolineForwardTlvs`] (trampoline
+/// node-based forwarding) implement this trait, allowing blinded path construction to be generic
+/// over the forwarding mechanism.
+///
+/// This trait is sealed and is not intended for implementation outside of this crate.
+pub trait ForwardTlvsInfo: Writeable + Clone + sealed::ForwardTlvsInfo {
+	/// The payment relay parameters for this hop.
+	fn payment_relay(&self) -> &PaymentRelay;
+	/// The payment constraints for this hop.
+	fn payment_constraints(&self) -> &PaymentConstraints;
+	/// The features for this hop.
+	fn features(&self) -> &BlindedHopFeatures;
+}
+
+/// An intermediate node, its forwarding parameters, and its [`ForwardTlvsInfo`] for use in a
+/// [`BlindedPaymentPath`].
 #[derive(Clone, Debug)]
-pub struct PaymentForwardNode {
+pub struct ForwardNode<F: ForwardTlvsInfo> {
 	/// The TLVs for this node's [`BlindedHop`], where the fee parameters contained within are also
 	/// used for [`BlindedPayInfo`] construction.
-	pub tlvs: ForwardTlvs,
+	pub tlvs: F,
 	/// This node's pubkey.
 	pub node_id: PublicKey,
 	/// The maximum value, in msat, that may be accepted by this node.
 	pub htlc_maximum_msat: u64,
 }
 
+/// An intermediate node for a regular (non-trampoline) [`BlindedPaymentPath`].
+pub type PaymentForwardNode = ForwardNode<ForwardTlvs>;
+
 /// Data to construct a [`BlindedHop`] for forwarding a payment.
 #[derive(Clone, Debug)]
 pub struct ForwardTlvs {
@@ -354,6 +382,20 @@ pub struct ForwardTlvs {
 	pub next_blinding_override: Option<PublicKey>,
 }
 
+impl sealed::ForwardTlvsInfo for ForwardTlvs {}
+
+impl ForwardTlvsInfo for ForwardTlvs {
+	fn payment_relay(&self) -> &PaymentRelay {
+		&self.payment_relay
+	}
+	fn payment_constraints(&self) -> &PaymentConstraints {
+		&self.payment_constraints
+	}
+	fn features(&self) -> &BlindedHopFeatures {
+		&self.features
+	}
+}
+
 /// Data to construct a [`BlindedHop`] for forwarding a Trampoline payment.
 #[derive(Clone, Debug)]
 pub struct TrampolineForwardTlvs {
@@ -373,6 +415,20 @@ pub struct TrampolineForwardTlvs {
 	pub next_blinding_override: Option<PublicKey>,
 }
 
+impl sealed::ForwardTlvsInfo for TrampolineForwardTlvs {}
+
+impl ForwardTlvsInfo for TrampolineForwardTlvs {
+	fn payment_relay(&self) -> &PaymentRelay {
+		&self.payment_relay
+	}
+	fn payment_constraints(&self) -> &PaymentConstraints {
+		&self.payment_constraints
+	}
+	fn features(&self) -> &BlindedHopFeatures {
+		&self.features
+	}
+}
+
 /// TLVs carried by a dummy hop within a blinded payment path.
 ///
 /// Dummy hops do not correspond to real forwarding decisions, but are processed
@@ -440,8 +496,8 @@ pub(crate) enum BlindedTrampolineTlvs {
 
 // Used to include forward and receive TLVs in the same iterator for encoding.
 #[derive(Clone)]
-enum BlindedPaymentTlvsRef<'a> {
-	Forward(&'a ForwardTlvs),
+enum BlindedPaymentTlvsRef<'a, F: ForwardTlvsInfo = ForwardTlvs> {
+	Forward(&'a F),
 	Dummy(&'a DummyTlvs),
 	Receive(&'a ReceiveTlvs),
 }
@@ -619,7 +675,7 @@ impl Writeable for ReceiveTlvs {
 	}
 }
 
-impl<'a> Writeable for BlindedPaymentTlvsRef<'a> {
+impl<'a, F: ForwardTlvsInfo> Writeable for BlindedPaymentTlvsRef<'a, F> {
 	fn write<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> {
 		match self {
 			Self::Forward(tlvs) => tlvs.write(w)?,
@@ -723,8 +779,8 @@ impl Readable for BlindedTrampolineTlvs {
 pub(crate) const PAYMENT_PADDING_ROUND_OFF: usize = 30;
 
 /// Construct blinded payment hops for the given `intermediate_nodes` and payee info.
-pub(super) fn blinded_hops<T: secp256k1::Signing + secp256k1::Verification>(
-	secp_ctx: &Secp256k1<T>, intermediate_nodes: &[PaymentForwardNode], payee_node_id: PublicKey,
+pub(super) fn blinded_hops<F: ForwardTlvsInfo, T: secp256k1::Signing + secp256k1::Verification>(
+	secp_ctx: &Secp256k1<T>, intermediate_nodes: &[ForwardNode<F>], payee_node_id: PublicKey,
 	dummy_tlvs: &[DummyTlvs], payee_tlvs: ReceiveTlvs, session_priv: &SecretKey,
 	local_node_receive_key: ReceiveAuthKey,
 ) -> Vec<BlindedHop> {
@@ -823,15 +879,15 @@ where
 	Ok((curr_base_fee, curr_prop_mil))
 }
 
-pub(super) fn compute_payinfo(
-	intermediate_nodes: &[PaymentForwardNode], dummy_tlvs: &[DummyTlvs], payee_tlvs: &ReceiveTlvs,
+pub(super) fn compute_payinfo<F: ForwardTlvsInfo>(
+	intermediate_nodes: &[ForwardNode<F>], dummy_tlvs: &[DummyTlvs], payee_tlvs: &ReceiveTlvs,
 	payee_htlc_maximum_msat: u64, min_final_cltv_expiry_delta: u16,
 ) -> Result<BlindedPayInfo, ()> {
 	let routing_fees = intermediate_nodes
 		.iter()
 		.map(|node| RoutingFees {
-			base_msat: node.tlvs.payment_relay.fee_base_msat,
-			proportional_millionths: node.tlvs.payment_relay.fee_proportional_millionths,
+			base_msat: node.tlvs.payment_relay().fee_base_msat,
+			proportional_millionths: node.tlvs.payment_relay().fee_proportional_millionths,
 		})
 		.chain(dummy_tlvs.iter().map(|tlvs| RoutingFees {
 			base_msat: tlvs.payment_relay.fee_base_msat,
@@ -847,24 +903,24 @@ pub(super) fn compute_payinfo(
 	for node in intermediate_nodes.iter() {
 		// In the future, we'll want to take the intersection of all supported features for the
 		// `BlindedPayInfo`, but there are no features in that context right now.
-		if node.tlvs.features.requires_unknown_bits_from(&BlindedHopFeatures::empty()) {
+		if node.tlvs.features().requires_unknown_bits_from(&BlindedHopFeatures::empty()) {
 			return Err(());
 		}
 
 		cltv_expiry_delta =
-			cltv_expiry_delta.checked_add(node.tlvs.payment_relay.cltv_expiry_delta).ok_or(())?;
+			cltv_expiry_delta.checked_add(node.tlvs.payment_relay().cltv_expiry_delta).ok_or(())?;
 
 		// The min htlc for an intermediate node is that node's min minus the fees charged by all of the
 		// following hops for forwarding that min, since that fee amount will automatically be included
 		// in the amount that this node receives and contribute towards reaching its min.
 		htlc_minimum_msat = amt_to_forward_msat(
-			core::cmp::max(node.tlvs.payment_constraints.htlc_minimum_msat, htlc_minimum_msat),
-			&node.tlvs.payment_relay,
+			core::cmp::max(node.tlvs.payment_constraints().htlc_minimum_msat, htlc_minimum_msat),
+			node.tlvs.payment_relay(),
 		)
 		.unwrap_or(1); // If underflow occurs, we definitely reached this node's min
 		htlc_maximum_msat = amt_to_forward_msat(
 			core::cmp::min(node.htlc_maximum_msat, htlc_maximum_msat),
-			&node.tlvs.payment_relay,
+			node.tlvs.payment_relay(),
 		)
 		.ok_or(())?; // If underflow occurs, we cannot send to this hop without exceeding their max
 	}
@@ -1038,8 +1094,14 @@ mod tests {
 			payment_constraints: PaymentConstraints { max_cltv_expiry: 0, htlc_minimum_msat: 1 },
 			payment_context: PaymentContext::Bolt12Refund(Bolt12RefundContext {}),
 		};
-		let blinded_payinfo =
-			super::compute_payinfo(&[], &[], &recv_tlvs, 4242, TEST_FINAL_CLTV as u16).unwrap();
+		let blinded_payinfo = super::compute_payinfo::<ForwardTlvs>(
+			&[],
+			&[],
+			&recv_tlvs,
+			4242,
+			TEST_FINAL_CLTV as u16,
+		)
+		.unwrap();
 		assert_eq!(blinded_payinfo.fee_base_msat, 0);
 		assert_eq!(blinded_payinfo.fee_proportional_millionths, 0);
 		assert_eq!(blinded_payinfo.cltv_expiry_delta, TEST_FINAL_CLTV as u16);