multi: represent the blinded forwarding next hop as an fn.Either

diff --git a/contractcourt/htlc_incoming_contest_resolver.go b/contractcourt/htlc_incoming_contest_resolver.go
index 3ee8cd6..c1a289a 100644
--- a/contractcourt/htlc_incoming_contest_resolver.go
+++ b/contractcourt/htlc_incoming_contest_resolver.go
@@ -84,7 +84,7 @@ func (h *htlcIncomingContestResolver) processFinalHtlcFail() error {
 func (h *htlcIncomingContestResolver) invalidFinalHtlc(
 	payload *hop.Payload, height uint32) bool {
 
-	if payload.FwdInfo.NextHop != hop.Exit {
+	if !payload.FwdInfo.IsExit() {
 		return false
 	}
 
@@ -312,7 +312,7 @@ func (h *htlcIncomingContestResolver) Resolve() (ContractResolver, error) {
 		hodlChan       <-chan interface{}
 		witnessUpdates <-chan lntypes.Preimage
 	)
-	if payload.FwdInfo.NextHop == hop.Exit {
+	if payload.FwdInfo.IsExit() {
 		// Create a buffered hodl chan to prevent deadlock.
 		hodlQueue := queue.NewConcurrentQueue(10)
 		hodlQueue.Start()
@@ -701,7 +701,7 @@ func (h *htlcIncomingContestResolver) findAndapplyPreimage() (bool, error) {
 
 	// Exit early if this is not the exit hop, which means we are not the
 	// payment receiver and don't have the preimage.
-	if payload.FwdInfo.NextHop != hop.Exit {
+	if !payload.FwdInfo.IsExit() {
 		return false, nil
 	}
 
diff --git a/htlcswitch/hop/forwarding_info.go b/htlcswitch/hop/forwarding_info.go
index f46b924..8e05872 100644
--- a/htlcswitch/hop/forwarding_info.go
+++ b/htlcswitch/hop/forwarding_info.go
@@ -2,6 +2,7 @@ package hop
 
 import (
 	"github.com/btcsuite/btcd/chainhash/v2"
+	"github.com/lightningnetwork/lnd/fn/v2"
 	"github.com/lightningnetwork/lnd/lnwire"
 )
 
@@ -11,10 +12,14 @@ import (
 // received within the incoming HTLC, to ensure that the prior hop didn't
 // tamper with the end-to-end routing information at all.
 type ForwardingInfo struct {
-	// NextHop is the channel ID of the next hop. The received HTLC should
-	// be forwarded to this particular channel in order to continue the
-	// end-to-end route.
-	NextHop lnwire.ShortChannelID
+	// NextHop identifies the next hop the HTLC should be forwarded to. In
+	// the common case it is a Left holding the short channel ID of the
+	// outgoing channel. For a blinded route whose recipient identifies the
+	// next hop by node ID (next_node_id) it is a Right holding the next
+	// node's compressed public key, which the switch's non-strict
+	// forwarding logic resolves to one of our channels with that peer. The
+	// zero value is a Left equal to hop.Exit, which denotes the exit hop.
+	NextHop fn.Either[lnwire.ShortChannelID, [33]byte]
 
 	// AmountToForward is the amount of milli-satoshis that the receiving
 	// node should forward to the next hop.
@@ -35,6 +40,35 @@ type ForwardingInfo struct {
 	PathID *chainhash.Hash
 }
 
+// NewChannelNextHop returns a next-hop value that identifies the outgoing
+// channel by its short channel ID, which is the common case.
+func NewChannelNextHop(
+	scid lnwire.ShortChannelID) fn.Either[lnwire.ShortChannelID, [33]byte] {
+
+	return fn.NewLeft[lnwire.ShortChannelID, [33]byte](scid)
+}
+
+// IsExit returns true if this forwarding info denotes the exit hop, i.e. we are
+// the final recipient of the HTLC. This is the case when the next hop is a
+// short channel ID equal to hop.Exit. A node-ID next hop (used by some blinded
+// routes) is always a forward, never the exit hop.
+func (f ForwardingInfo) IsExit() bool {
+	var isExit bool
+	f.NextHop.WhenLeft(func(scid lnwire.ShortChannelID) {
+		isExit = scid == Exit
+	})
+
+	return isExit
+}
+
+// NextHopChannel returns the short channel ID of the outgoing channel when the
+// next hop is identified by channel ID (the common case). It returns None when
+// the next hop is identified by node ID instead, in which case the outgoing
+// channel is selected by the switch's non-strict forwarding.
+func (f ForwardingInfo) NextHopChannel() fn.Option[lnwire.ShortChannelID] {
+	return f.NextHop.LeftToSome()
+}
+
 // FinalHtlcValidationResult describes the result of checking a final-hop
 // HTLC against the onion payload and supported final-hop CLTV range.
 type FinalHtlcValidationResult uint8
diff --git a/htlcswitch/hop/forwarding_info_test.go b/htlcswitch/hop/forwarding_info_test.go
index 82a5ad0..284c7c3 100644
--- a/htlcswitch/hop/forwarding_info_test.go
+++ b/htlcswitch/hop/forwarding_info_test.go
@@ -21,7 +21,7 @@ func TestValidateFinalHtlc(t *testing.T) {
 	fwdInfo := ForwardingInfo{
 		AmountToForward: amount,
 		OutgoingCLTV:    expiry,
-		NextHop:         Exit,
+		NextHop:         NewChannelNextHop(Exit),
 	}
 
 	testCases := []struct {
@@ -115,7 +115,7 @@ func TestValidateFinalHtlc(t *testing.T) {
 		fwdInfo: ForwardingInfo{
 			AmountToForward: amount,
 			OutgoingCLTV:    expiry + maxCltvDelta + 2,
-			NextHop:         Exit,
+			NextHop:         NewChannelNextHop(Exit),
 		},
 		validateAmount: true,
 		expected:       FinalHtlcInvalidCltv,
diff --git a/htlcswitch/hop/fuzz_test.go b/htlcswitch/hop/fuzz_test.go
index bafede0..cbb4619 100644
--- a/htlcswitch/hop/fuzz_test.go
+++ b/htlcswitch/hop/fuzz_test.go
@@ -96,7 +96,7 @@ func hopFromPayload(p *Payload) (*route.Hop, uint64) {
 		BlindingPoint:    p.blindingPoint,
 		CustomRecords:    p.customRecords,
 		TotalAmtMsat:     p.totalAmtMsat,
-	}, p.FwdInfo.NextHop.ToUint64()
+	}, p.FwdInfo.NextHop.UnwrapLeftOr(Exit).ToUint64()
 }
 
 // FuzzPayloadFinal fuzzes final hop payloads, providing the additional context
diff --git a/htlcswitch/hop/iterator.go b/htlcswitch/hop/iterator.go
index ada071e..1ef559f 100644
--- a/htlcswitch/hop/iterator.go
+++ b/htlcswitch/hop/iterator.go
@@ -324,8 +324,9 @@ func deriveBlindedRouteForwardingInfo(r *sphinxHopIterator,
 	if err != nil {
 		return nil, routeRole, err
 	}
+
 	payload.FwdInfo = ForwardingInfo{
-		NextHop:         nextSCID.Val,
+		NextHop:         NewChannelNextHop(nextSCID.Val),
 		AmountToForward: fwdAmt,
 		OutgoingCLTV: r.blindingKit.IncomingCltv - uint32(
 			relayInfo.Val.CltvExpiryDelta,
diff --git a/htlcswitch/hop/iterator_test.go b/htlcswitch/hop/iterator_test.go
index e60aa16..7e6b030 100644
--- a/htlcswitch/hop/iterator_test.go
+++ b/htlcswitch/hop/iterator_test.go
@@ -33,7 +33,9 @@ func TestSphinxHopIteratorForwardingInstructions(t *testing.T) {
 	// extract each type, no matter the payload type.
 	nextAddrInt := binary.BigEndian.Uint64(hopData.NextAddress[:])
 	expectedFwdInfo := ForwardingInfo{
-		NextHop:         lnwire.NewShortChanIDFromInt(nextAddrInt),
+		NextHop: NewChannelNextHop(
+			lnwire.NewShortChanIDFromInt(nextAddrInt),
+		),
 		AmountToForward: lnwire.MilliSatoshi(hopData.ForwardAmount),
 		OutgoingCLTV:    hopData.OutgoingCltv,
 	}
diff --git a/htlcswitch/hop/payload.go b/htlcswitch/hop/payload.go
index dccbc3a..5f60467 100644
--- a/htlcswitch/hop/payload.go
+++ b/htlcswitch/hop/payload.go
@@ -126,7 +126,9 @@ func NewLegacyPayload(f *sphinx.HopData) *Payload {
 
 	return &Payload{
 		FwdInfo: ForwardingInfo{
-			NextHop:         lnwire.NewShortChanIDFromInt(nextHop),
+			NextHop: NewChannelNextHop(
+				lnwire.NewShortChanIDFromInt(nextHop),
+			),
 			AmountToForward: lnwire.MilliSatoshi(f.ForwardAmount),
 			OutgoingCLTV:    f.OutgoingCltv,
 		},
@@ -201,7 +203,9 @@ func ParseTLVPayload(r io.Reader) (*Payload, map[tlv.Type][]byte, error) {
 
 	return &Payload{
 		FwdInfo: ForwardingInfo{
-			NextHop:         lnwire.NewShortChanIDFromInt(cid),
+			NextHop: NewChannelNextHop(
+				lnwire.NewShortChanIDFromInt(cid),
+			),
 			AmountToForward: lnwire.MilliSatoshi(amt),
 			OutgoingCLTV:    cltv,
 		},
diff --git a/htlcswitch/link.go b/htlcswitch/link.go
index 715afe7..5ce8160 100644
--- a/htlcswitch/link.go
+++ b/htlcswitch/link.go
@@ -3189,8 +3189,8 @@ func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg) {
 			continue
 		}
 
-		switch fwdInfo.NextHop {
-		case hop.Exit:
+		switch {
+		case fwdInfo.IsExit():
 			err := l.processExitHop(
 				add, sourceRef, obfuscator, fwdInfo,
 				heightNow, pld,
@@ -3268,7 +3268,7 @@ func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg) {
 				updatePacket := &htlcPacket{
 					incomingChanID:       l.ShortChanID(),
 					incomingHTLCID:       add.ID,
-					outgoingChanID:       fwdInfo.NextHop,
+					outgoingChanID:       fwdInfo.NextHopChannel().UnwrapOr(hop.Exit),
 					sourceRef:            &sourceRef,
 					incomingAmount:       add.Amount,
 					amount:               outgoingAdd.Amount,
@@ -3345,7 +3345,7 @@ func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg) {
 				updatePacket := &htlcPacket{
 					incomingChanID:       l.ShortChanID(),
 					incomingHTLCID:       add.ID,
-					outgoingChanID:       fwdInfo.NextHop,
+					outgoingChanID:       fwdInfo.NextHopChannel().UnwrapOr(hop.Exit),
 					sourceRef:            &sourceRef,
 					incomingAmount:       add.Amount,
 					amount:               addMsg.Amount,
diff --git a/htlcswitch/link_test.go b/htlcswitch/link_test.go
index 72e08f3..2e5fdfb 100644
--- a/htlcswitch/link_test.go
+++ b/htlcswitch/link_test.go
@@ -777,8 +777,9 @@ func testChannelLinkInboundFee(t *testing.T, //nolint:thelper
 	hops := []*hop.Payload{
 		{
 			FwdInfo: hop.ForwardingInfo{
-				NextHop: n.carolChannelLink.
-					ShortChanID(),
+				NextHop: hop.NewChannelNextHop(
+					n.carolChannelLink.ShortChanID(),
+				),
 				AmountToForward: 1_000_000,
 				OutgoingCLTV:    106,
 			},
diff --git a/htlcswitch/mock.go b/htlcswitch/mock.go
index a3079e6..9d201d5 100644
--- a/htlcswitch/mock.go
+++ b/htlcswitch/mock.go
@@ -368,7 +368,13 @@ func (r *mockHopIterator) EncodeNextHop(w io.Writer) error {
 }
 
 func encodeFwdInfo(w io.Writer, f *hop.ForwardingInfo) error {
-	if err := binary.Write(w, binary.BigEndian, f.NextHop); err != nil {
+	if f.NextHop.IsRight() {
+		return fmt.Errorf("mock serialization does not support " +
+			"node-ID next hop")
+	}
+
+	nextHop := f.NextHopChannel().UnwrapOr(hop.Exit)
+	if err := binary.Write(w, binary.BigEndian, nextHop); err != nil {
 		return err
 	}
 
@@ -510,7 +516,8 @@ func (p *mockIteratorDecoder) DecodeHopIterator(r io.Reader, rHash []byte,
 		}
 
 		var nextHopBytes [8]byte
-		binary.BigEndian.PutUint64(nextHopBytes[:], f.NextHop.ToUint64())
+		scid := f.NextHopChannel().UnwrapOr(hop.Exit)
+		binary.BigEndian.PutUint64(nextHopBytes[:], scid.ToUint64())
 
 		hops[i] = hop.NewLegacyPayload(&sphinx.HopData{
 			Realm:         [1]byte{}, // hop.BitcoinNetwork
@@ -563,9 +570,11 @@ func (p *mockIteratorDecoder) DecodeHopIterators(id []byte,
 }
 
 func decodeFwdInfo(r io.Reader, f *hop.ForwardingInfo) error {
-	if err := binary.Read(r, binary.BigEndian, &f.NextHop); err != nil {
+	var nextHop lnwire.ShortChannelID
+	if err := binary.Read(r, binary.BigEndian, &nextHop); err != nil {
 		return err
 	}
+	f.NextHop = hop.NewChannelNextHop(nextHop)
 
 	if err := binary.Read(r, binary.BigEndian, &f.AmountToForward); err != nil {
 		return err
diff --git a/routing/pathfind_test.go b/routing/pathfind_test.go
index 6b6d2cc..94942fc 100644
--- a/routing/pathfind_test.go
+++ b/routing/pathfind_test.go
@@ -1190,7 +1190,9 @@ func testBasicGraphPathFindingCase(t *testing.T, graphInstance *testGraphInstanc
 
 		require.Equal(
 			t, route.Hops[i+1].ChannelID,
-			payload.FwdInfo.NextHop.ToUint64(),
+			payload.FwdInfo.NextHopChannel().UnwrapOr(
+				switchhop.Exit,
+			).ToUint64(),
 		)
 	}
 
@@ -1203,7 +1205,11 @@ func testBasicGraphPathFindingCase(t *testing.T, graphInstance *testGraphInstanc
 
 	// The final hop should have a next hop value of all zeroes in order
 	// to indicate it's the exit hop.
-	require.Zero(t, payload.FwdInfo.NextHop.ToUint64())
+	require.Zero(
+		t, payload.FwdInfo.NextHopChannel().UnwrapOr(
+			switchhop.Exit,
+		).ToUint64(),
+	)
 
 	var expectedTotalFee lnwire.MilliSatoshi
 	for i := 0; i < expectedHopCount; i++ {
diff --git a/witness_beacon.go b/witness_beacon.go
index c1ccc63..1c2e78c 100644
--- a/witness_beacon.go
+++ b/witness_beacon.go
@@ -114,7 +114,7 @@ func (p *preimageBeacon) SubscribeUpdates(
 		IncomingExpiry:       htlc.RefundTimeout,
 		IncomingAmount:       htlc.Amt,
 		IncomingCircuit:      inKey,
-		OutgoingChanID:       payload.FwdInfo.NextHop,
+		OutgoingChanID:       payload.FwdInfo.NextHopChannel().UnwrapOr(hop.Exit),
 		OutgoingExpiry:       payload.FwdInfo.OutgoingCLTV,
 		OutgoingAmount:       payload.FwdInfo.AmountToForward,
 		InOnionCustomRecords: payload.CustomRecords(),