htlcswitch+peer: set and read aux custom records

diff --git a/htlcswitch/link.go b/htlcswitch/link.go
index 2d1dd7d..4c81964 100644
--- a/htlcswitch/link.go
+++ b/htlcswitch/link.go
@@ -298,6 +298,11 @@ type ChannelLinkConfig struct {
 	// used to manage the bandwidth of the link.
 	AuxTrafficShaper fn.Option[AuxTrafficShaper]
 
+	// AuxChannelNegotiator is an optional interface that allows aux channel
+	// implementations to inject and process custom records over channel
+	// related wire messages.
+	AuxChannelNegotiator fn.Option[lnwallet.AuxChannelNegotiator]
+
 	// QuiescenceTimeout is the max duration that the channel can be
 	// quiesced. Any dependent protocols (dynamic commitments, splicing,
 	// etc.) must finish their operations under this timeout value,
@@ -987,6 +992,22 @@ func (l *channelLink) syncChanStates(ctx context.Context) error {
 		// In any case, we'll then process their ChanSync message.
 		l.log.Info("received re-establishment message from remote side")
 
+		// If we have an AuxChannelNegotiator we notify any external
+		// component for this message. This serves as a notification
+		// that the reestablish message was received.
+		l.cfg.AuxChannelNegotiator.WhenSome(
+			func(acn lnwallet.AuxChannelNegotiator) {
+				fundingPoint := l.channel.ChannelPoint()
+				cid := lnwire.NewChanIDFromOutPoint(
+					fundingPoint,
+				)
+
+				acn.ProcessReestablish(
+					cid, l.cfg.Peer.PubKey(),
+				)
+			},
+		)
+
 		var (
 			openedCircuits []CircuitKey
 			closedCircuits []CircuitKey
diff --git a/peer/brontide.go b/peer/brontide.go
index 57e340f..1ea16d1 100644
--- a/peer/brontide.go
+++ b/peer/brontide.go
@@ -456,6 +456,11 @@ type Config struct {
 	// used to modify the way the co-op close transaction is constructed.
 	AuxChanCloser fn.Option[chancloser.AuxChanCloser]
 
+	// AuxChannelNegotiator is an optional interface that allows aux channel
+	// implementations to inject and process custom records over channel
+	// related wire messages.
+	AuxChannelNegotiator fn.Option[lnwallet.AuxChannelNegotiator]
+
 	// ShouldFwdExpEndorsement is a closure that indicates whether
 	// experimental endorsement signals should be set.
 	ShouldFwdExpEndorsement func() bool
@@ -1454,8 +1459,9 @@ func (p *Brontide) addLink(chanPoint *wire.OutPoint,
 		ShouldFwdExpEndorsement: p.cfg.ShouldFwdExpEndorsement,
 		DisallowQuiescence: p.cfg.DisallowQuiescence ||
 			!p.remoteFeatures.HasFeature(lnwire.QuiescenceOptional),
-		AuxTrafficShaper:  p.cfg.AuxTrafficShaper,
-		QuiescenceTimeout: p.cfg.QuiescenceTimeout,
+		AuxTrafficShaper:     p.cfg.AuxTrafficShaper,
+		AuxChannelNegotiator: p.cfg.AuxChannelNegotiator,
+		QuiescenceTimeout:    p.cfg.QuiescenceTimeout,
 	}
 
 	// Before adding our new link, purge the switch of any pending or live
@@ -4537,6 +4543,19 @@ func (p *Brontide) handleInitMsg(msg *lnwire.Init) error {
 		return fmt.Errorf("data loss protection required")
 	}
 
+	// If we have an AuxChannelNegotiator and the peer sent aux features,
+	// process them.
+	p.cfg.AuxChannelNegotiator.WhenSome(
+		func(acn lnwallet.AuxChannelNegotiator) {
+			err = acn.ProcessInitRecords(
+				p.cfg.PubKeyBytes, msg.CustomRecords.Copy(),
+			)
+		},
+	)
+	if err != nil {
+		return fmt.Errorf("could not process init records: %w", err)
+	}
+
 	return nil
 }
 
@@ -4597,6 +4616,30 @@ func (p *Brontide) sendInitMsg(legacyChan bool) error {
 		features.RawFeatureVector,
 	)
 
+	var err error
+
+	// If we have an AuxChannelNegotiator, get custom feature bits to
+	// include in the init message.
+	p.cfg.AuxChannelNegotiator.WhenSome(
+		func(negotiator lnwallet.AuxChannelNegotiator) {
+			var auxRecords lnwire.CustomRecords
+			auxRecords, err = negotiator.GetInitRecords(
+				p.cfg.PubKeyBytes,
+			)
+			if err != nil {
+				p.log.Warnf("Failed to get aux init features: "+
+					"%v", err)
+				return
+			}
+
+			mergedRecs := msg.CustomRecords.MergedCopy(auxRecords)
+			msg.CustomRecords = mergedRecs
+		},
+	)
+	if err != nil {
+		return err
+	}
+
 	return p.writeMessage(msg)
 }
 
diff --git a/server.go b/server.go
index 44be180..33b8fd7 100644
--- a/server.go
+++ b/server.go
@@ -4431,6 +4431,7 @@ func (s *server) peerConnected(conn net.Conn, connReq *connmgr.ConnReq,
 		AuxChanCloser:          s.implCfg.AuxChanCloser,
 		AuxResolver:            s.implCfg.AuxContractResolver,
 		AuxTrafficShaper:       s.implCfg.TrafficShaper,
+		AuxChannelNegotiator:   s.implCfg.AuxChannelNegotiator,
 		ShouldFwdExpEndorsement: func() bool {
 			if s.cfg.ProtocolOptions.NoExperimentalEndorsement() {
 				return false