htlcswitch: add NextBlinding to ForwardingInfo and set in UpdateAddHtlc

When we have a HTLC that is part of a blinded route, we need to include
the next ephemeral blinding point in UpdateAddHtlc for the next hop. The
way that we handle the addition of this key is the same for introduction
nodes and relaying nodes within the route.

htlcswitch/hop/iterator.go

@@ -131,6 +131,10 @@ type BlindingProcessor interface {
 	// ephemeral key provided.
 	DecryptBlindedHopData(ephemPub *btcec.PublicKey,
 		encryptedData []byte) ([]byte, error)
+
+	// NextEphemeral returns the next hop's ephemeral key, calculated
+	// from the current ephemeral key provided.
+	NextEphemeral(*btcec.PublicKey) (*btcec.PublicKey, error)
 }
 
 // BlindingKit contains the components required to extract forwarding
@@ -250,12 +254,40 @@ func (b *BlindingKit) DecryptAndValidateFwdInfo(payload *Payload,
 		return nil, err
 	}
 
+	// If we have an override for the blinding point for the next node,
+	// we'll just use it without tweaking (the sender intended to switch
+	// out directly for this blinding point). Otherwise, we'll tweak our
+	// blinding point to get the next ephemeral key.
+	nextEph, err := routeData.NextBlindingOverride.UnwrapOrFuncErr(
+		func() (tlv.RecordT[tlv.TlvType8,
+			*btcec.PublicKey], error) {
+
+			next, err := b.Processor.NextEphemeral(blindingPoint)
+			if err != nil {
+				// Return a zero record because we expect the
+				// error to be checked.
+				return routeData.NextBlindingOverride.Zero(),
+					err
+			}
+
+			return tlv.NewPrimitiveRecord[tlv.TlvType8](next), nil
+		},
+	)
+	if err != nil {
+		return nil, err
+	}
+
 	return &ForwardingInfo{
 		NextHop:         routeData.ShortChannelID.Val,
 		AmountToForward: fwdAmt,
 		OutgoingCTLV: b.IncomingCltv - uint32(
 			routeData.RelayInfo.Val.CltvExpiryDelta,
 		),
+		// Remap from blinding override type to blinding point type.
+		NextBlinding: tlv.SomeRecordT(
+			tlv.NewPrimitiveRecord[lnwire.BlindingPointTlvType](
+				nextEph.Val),
+		),
 	}, nil
 }