chanstate: add remote commit chain store

diff --git a/channeldb/channel.go b/channeldb/channel.go
index f509d50..601fe47 100644
--- a/channeldb/channel.go
+++ b/channeldb/channel.go
@@ -2933,11 +2933,20 @@ func (c *OpenChannel) AppendRemoteCommitChain(diff *CommitDiff) error {
 		return ErrNoRestoredChannelMutation
 	}
 
-	return kvdb.Update(c.Db.backend, func(tx kvdb.RwTx) error {
+	return c.Db.AppendRemoteCommitChain(c, diff)
+}
+
+// AppendRemoteCommitChain appends a new CommitDiff to the remote party's
+// commitment chain.
+func (c *ChannelStateDB) AppendRemoteCommitChain(channel *OpenChannel,
+	diff *CommitDiff) error {
+
+	return kvdb.Update(c.backend, func(tx kvdb.RwTx) error {
 		// First, we'll grab the writable bucket where this channel's
 		// data resides.
 		chanBucket, err := fetchChanBucketRw(
-			tx, c.IdentityPub, &c.FundingOutpoint, c.ChainHash,
+			tx, channel.IdentityPub, &channel.FundingOutpoint,
+			channel.ChainHash,
 		)
 		if err != nil {
 			return err
@@ -2945,7 +2954,7 @@ func (c *OpenChannel) AppendRemoteCommitChain(diff *CommitDiff) error {
 
 		// If the channel is marked as borked, then for safety reasons,
 		// we shouldn't attempt any further updates.
-		isBorked, err := c.isBorked(chanBucket)
+		isBorked, err := channel.isBorked(chanBucket)
 		if err != nil {
 			return err
 		}
@@ -2958,7 +2967,7 @@ func (c *OpenChannel) AppendRemoteCommitChain(diff *CommitDiff) error {
 		// Mark all of these as being fully processed in our forwarding
 		// package, which prevents us from reprocessing them after
 		// startup.
-		err = c.Packager.AckAddHtlcs(tx, diff.AddAcks...)
+		err = channel.Packager.AckAddHtlcs(tx, diff.AddAcks...)
 		if err != nil {
 			return err
 		}
@@ -2968,7 +2977,9 @@ func (c *OpenChannel) AppendRemoteCommitChain(diff *CommitDiff) error {
 		// prevents the same fails and settles from being retransmitted
 		// after restarts. The actual fail or settle we need to
 		// propagate to the remote party is now in the commit diff.
-		err = c.Packager.AckSettleFails(tx, diff.SettleFailAcks...)
+		err = channel.Packager.AckSettleFails(
+			tx, diff.SettleFailAcks...,
+		)
 		if err != nil {
 			return err
 		}
diff --git a/chanstate/interface.go b/chanstate/interface.go
index 3e26f2d..323aa9c 100644
--- a/chanstate/interface.go
+++ b/chanstate/interface.go
@@ -232,6 +232,11 @@ type OpenChannelCommitmentStore[Channel any] interface {
 	UpdateChannelCommitment(channel Channel,
 		newCommitment *ChannelCommitment,
 		unsignedAckedUpdates []LogUpdate) (map[uint64]bool, error)
+
+	// AppendRemoteCommitChain appends a new CommitDiff to the remote
+	// party's commitment chain. This is used after preparing a new remote
+	// commitment state, before transmitting it to the remote party.
+	AppendRemoteCommitChain(channel Channel, diff *CommitDiff) error
 }
 
 // ClosedChannelStore owns closed-channel summaries and lifecycle mutations.