channeldb: store channel state by interface

diff --git a/channeldb/channel.go b/channeldb/channel.go
index 15bc802..c47d7e3 100644
--- a/channeldb/channel.go
+++ b/channeldb/channel.go
@@ -801,8 +801,11 @@ type OpenChannel struct {
 	// immutable.
 	CustomBlob fn.Option[tlv.Blob]
 
-	// TODO(roasbeef): eww
-	Db *ChannelStateDB
+	// Db persists channel state through the chanstate Store contract. This
+	// field intentionally keeps the existing name while the code moves from
+	// channeldb toward chanstate so call sites can become backend
+	// independent before the OpenChannel type itself is moved.
+	Db cstate.Store[*OpenChannel]
 
 	// TODO(roasbeef): just need to store local and remote HTLC's?
 
diff --git a/channeldb/close_channel_test.go b/channeldb/close_channel_test.go
index cd9c796..e2dcc63 100644
--- a/channeldb/close_channel_test.go
+++ b/channeldb/close_channel_test.go
@@ -15,10 +15,12 @@ import (
 // revocationLogBucket of the given channel. The helper navigates the raw KV
 // tree so the test does not depend on the higher-level commit-chain
 // machinery.
-func writeTestRevlogEntries(t *testing.T, ch *OpenChannel, n int) {
+func writeTestRevlogEntries(t *testing.T, cdb *ChannelStateDB,
+	ch *OpenChannel, n int) {
+
 	t.Helper()
 
-	err := kvdb.Update(ch.Db.backend, func(tx kvdb.RwTx) error {
+	err := kvdb.Update(cdb.backend, func(tx kvdb.RwTx) error {
 		openChanBkt := tx.ReadWriteBucket(openChannelBucket)
 		require.NotNil(t, openChanBkt, "openChannelBucket missing")
 
@@ -56,11 +58,13 @@ func writeTestRevlogEntries(t *testing.T, ch *OpenChannel, n int) {
 
 // writeTestForwardingPackages writes n empty forwarding packages for the
 // given channel using distinct remote commitment heights.
-func writeTestForwardingPackages(t *testing.T, ch *OpenChannel, n int) {
+func writeTestForwardingPackages(t *testing.T, cdb *ChannelStateDB,
+	ch *OpenChannel, n int) {
+
 	t.Helper()
 
 	packager := NewChannelPackager(ch.ShortChanID())
-	err := kvdb.Update(ch.Db.backend, func(tx kvdb.RwTx) error {
+	err := kvdb.Update(cdb.backend, func(tx kvdb.RwTx) error {
 		for i := range n {
 			pkg := NewFwdPkg(
 				ch.ShortChanID(), uint64(i), nil, nil,
@@ -78,11 +82,13 @@ func writeTestForwardingPackages(t *testing.T, ch *OpenChannel, n int) {
 // countRevlogEntries returns the number of entries in the revocationLogBucket
 // for the given channel, or -1 if the channel bucket no longer exists in
 // openChannelBucket.
-func countRevlogEntries(t *testing.T, ch *OpenChannel) int {
+func countRevlogEntries(t *testing.T, cdb *ChannelStateDB,
+	ch *OpenChannel) int {
+
 	t.Helper()
 
 	count := -1
-	err := kvdb.View(ch.Db.backend, func(tx kvdb.RTx) error {
+	err := kvdb.View(cdb.backend, func(tx kvdb.RTx) error {
 		openChanBkt := tx.ReadBucket(openChannelBucket)
 		if openChanBkt == nil {
 			return nil
@@ -202,8 +208,8 @@ func TestCloseChannelTombstoneWritePath(t *testing.T) {
 
 	const numRevlogEntries = 5
 	const numFwdPkgs = 3
-	writeTestRevlogEntries(t, ch, numRevlogEntries)
-	writeTestForwardingPackages(t, ch, numFwdPkgs)
+	writeTestRevlogEntries(t, cdb, ch, numRevlogEntries)
+	writeTestForwardingPackages(t, cdb, ch, numFwdPkgs)
 
 	closeChannelForTest(t, cdb, ch)
 
@@ -224,7 +230,7 @@ func TestCloseChannelTombstoneWritePath(t *testing.T) {
 	require.Equal(t, ch.FundingOutpoint, closeSummary.ChanPoint)
 
 	// Bulk state preserved on disk — tombstoning's whole point.
-	require.Equal(t, numRevlogEntries, countRevlogEntries(t, ch))
+	require.Equal(t, numRevlogEntries, countRevlogEntries(t, cdb, ch))
 
 	packager := NewChannelPackager(ch.ShortChanID())
 	var fwdPkgs []*FwdPkg
@@ -281,7 +287,7 @@ func TestCloseChannelTombstoneRemovesFromOpenScans(t *testing.T) {
 	ch2 := createTestChannel(t, cdb, openChannelOption())
 
 	const numRevlogEntries = 5
-	writeTestRevlogEntries(t, ch1, numRevlogEntries)
+	writeTestRevlogEntries(t, cdb, ch1, numRevlogEntries)
 
 	openChans, err := cdb.FetchAllChannels()
 	require.NoError(t, err)
@@ -313,7 +319,7 @@ func TestCloseChannelTombstoneRemovesFromOpenScans(t *testing.T) {
 
 	// The bulk historical state stays put — that is the whole point of
 	// the tombstone path on these backends.
-	require.Equal(t, numRevlogEntries, countRevlogEntries(t, ch1))
+	require.Equal(t, numRevlogEntries, countRevlogEntries(t, cdb, ch1))
 
 	// The outpoint index for ch1 must flip to closed; ch2's stays open.
 	require.Equal(t, outpointClosed, readOutpointStatus(
@@ -380,14 +386,14 @@ func TestCloseChannelSync(t *testing.T) {
 	ch := createTestChannel(t, cdb, openChannelOption())
 
 	const numRevlogEntries = 4
-	writeTestRevlogEntries(t, ch, numRevlogEntries)
-	writeTestForwardingPackages(t, ch, 3)
+	writeTestRevlogEntries(t, cdb, ch, numRevlogEntries)
+	writeTestForwardingPackages(t, cdb, ch, 3)
 
 	closeChannelForTest(t, cdb, ch)
 
 	// The synchronous path wipes the chanBucket inline, so
 	// countRevlogEntries must report -1 (bucket is gone, not just empty).
-	require.Equal(t, -1, countRevlogEntries(t, ch),
+	require.Equal(t, -1, countRevlogEntries(t, cdb, ch),
 		"channel bucket must be deleted after sync close")
 
 	// Forwarding packages are wiped inline.
diff --git a/contractcourt/breach_arbitrator_test.go b/contractcourt/breach_arbitrator_test.go
index 37cf079..a000a6c 100644
--- a/contractcourt/breach_arbitrator_test.go
+++ b/contractcourt/breach_arbitrator_test.go
@@ -952,7 +952,8 @@ func initBreachedState(t *testing.T) (*BreachArbitrator,
 	contractBreaches := make(chan *ContractBreachEvent)
 
 	brar, err := createTestArbiter(
-		t, contractBreaches, alice.State().Db.GetParentDB(),
+		t, contractBreaches,
+		testChannelStateDB(t, alice.State()).GetParentDB(),
 	)
 	require.NoError(t, err, "unable to initialize test breach arbiter")
 
@@ -1118,7 +1119,8 @@ func TestBreachHandoffFail(t *testing.T) {
 	assertNotPendingClosed(t, alice)
 
 	brar, err := createTestArbiter(
-		t, contractBreaches, alice.State().Db.GetParentDB(),
+		t, contractBreaches,
+		testChannelStateDB(t, alice.State()).GetParentDB(),
 	)
 	require.NoError(t, err, "unable to initialize test breach arbiter")
 
@@ -1763,7 +1765,9 @@ func testBreachSpends(t *testing.T, test breachTest) {
 	}
 
 	// Assert that the channel is fully resolved.
-	assertBrarCleanup(t, brar, &chanPoint, alice.State().Db)
+	assertBrarCleanup(
+		t, brar, &chanPoint, testChannelStateDB(t, alice.State()),
+	)
 }
 
 // TestBreachDelayedJusticeConfirmation tests that the breach arbiter will
@@ -1968,7 +1972,9 @@ func TestBreachDelayedJusticeConfirmation(t *testing.T) {
 	}
 
 	// Assert that the channel is fully resolved.
-	assertBrarCleanup(t, brar, &chanPoint, alice.State().Db)
+	assertBrarCleanup(
+		t, brar, &chanPoint, testChannelStateDB(t, alice.State()),
+	)
 }
 
 // findInputIndex returns the index of the input that spends from the given
@@ -2080,7 +2086,9 @@ func assertBrarCleanup(t *testing.T, brar *BreachArbitrator,
 func assertPendingClosed(t *testing.T, c *lnwallet.LightningChannel) {
 	t.Helper()
 
-	closedChans, err := c.State().Db.FetchClosedChannels(true)
+	closedChans, err := testChannelStateDB(
+		t, c.State(),
+	).FetchClosedChannels(true)
 	require.NoError(t, err, "unable to load pending closed channels")
 
 	for _, chanSummary := range closedChans {
@@ -2097,7 +2105,9 @@ func assertPendingClosed(t *testing.T, c *lnwallet.LightningChannel) {
 func assertNotPendingClosed(t *testing.T, c *lnwallet.LightningChannel) {
 	t.Helper()
 
-	closedChans, err := c.State().Db.FetchClosedChannels(true)
+	closedChans, err := testChannelStateDB(
+		t, c.State(),
+	).FetchClosedChannels(true)
 	require.NoError(t, err, "unable to load pending closed channels")
 
 	for _, chanSummary := range closedChans {
diff --git a/contractcourt/utils_test.go b/contractcourt/utils_test.go
index 994bc57..27b65b9 100644
--- a/contractcourt/utils_test.go
+++ b/contractcourt/utils_test.go
@@ -12,6 +12,20 @@ import (
 	"github.com/lightningnetwork/lnd/channeldb"
 )
 
+// testChannelStateDB extracts the ChannelStateDB from the test channel state.
+func testChannelStateDB(t testing.TB,
+	state *channeldb.OpenChannel) *channeldb.ChannelStateDB {
+
+	t.Helper()
+
+	cdb, ok := state.Db.(*channeldb.ChannelStateDB)
+	if !ok {
+		t.Fatalf("expected ChannelStateDB, got %T", state.Db)
+	}
+
+	return cdb
+}
+
 // timeout implements a test level timeout.
 func timeout() func() {
 	done := make(chan struct{})
@@ -56,7 +70,9 @@ func copyChannelState(t *testing.T, state *channeldb.OpenChannel) (
 	*channeldb.OpenChannel, error) {
 
 	// Make a copy of the DB.
-	dbFile := filepath.Join(state.Db.GetParentDB().Path(), "channel.db")
+	dbFile := filepath.Join(
+		testChannelStateDB(t, state).GetParentDB().Path(), "channel.db",
+	)
 	tempDbPath := t.TempDir()
 
 	tempDbFile := filepath.Join(tempDbPath, "channel.db")
diff --git a/htlcswitch/link_test.go b/htlcswitch/link_test.go
index fdc455f..573c0bb 100644
--- a/htlcswitch/link_test.go
+++ b/htlcswitch/link_test.go
@@ -2173,7 +2173,7 @@ func newSingleLinkTestHarness(t *testing.T, chanAmt,
 
 	pCache := newMockPreimageCache()
 
-	aliceDb := aliceLc.channel.State().Db.GetParentDB()
+	aliceDb := testChannelStateDB(t, aliceLc.channel).GetParentDB()
 	aliceSwitch, err := initSwitchWithDB(testStartingHeight, aliceDb)
 	if err != nil {
 		return singleLinkTestHarness{}, err
@@ -4853,7 +4853,7 @@ func (h *persistentLinkHarness) restartLink(
 		pCache = newMockPreimageCache()
 	)
 
-	aliceDb := aliceChannel.State().Db.GetParentDB()
+	aliceDb := testChannelStateDB(t, aliceChannel).GetParentDB()
 	if restartSwitch {
 		var err error
 		h.hSwitch, err = initSwitchWithDB(testStartingHeight, aliceDb)
diff --git a/htlcswitch/test_utils.go b/htlcswitch/test_utils.go
index 10f7bdb..110d97b 100644
--- a/htlcswitch/test_utils.go
+++ b/htlcswitch/test_utils.go
@@ -43,6 +43,20 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+// testChannelStateDB extracts the ChannelStateDB from the test channel.
+func testChannelStateDB(t testing.TB,
+	channel *lnwallet.LightningChannel) *channeldb.ChannelStateDB {
+
+	t.Helper()
+
+	cdb, ok := channel.State().Db.(*channeldb.ChannelStateDB)
+	if !ok {
+		t.Fatalf("expected ChannelStateDB, got %T", channel.State().Db)
+	}
+
+	return cdb
+}
+
 // maxInflightHtlcs specifies the max number of inflight HTLCs. This number is
 // chosen to be smaller than the default 483 so the test can run faster.
 const maxInflightHtlcs = 50
@@ -954,9 +968,9 @@ func newThreeHopNetwork(t testing.TB, aliceChannel, firstBobChannel,
 	secondBobChannel, carolChannel *lnwallet.LightningChannel,
 	startingHeight uint32, opts ...serverOption) *threeHopNetwork {
 
-	aliceDb := aliceChannel.State().Db.GetParentDB()
-	bobDb := firstBobChannel.State().Db.GetParentDB()
-	carolDb := carolChannel.State().Db.GetParentDB()
+	aliceDb := testChannelStateDB(t, aliceChannel).GetParentDB()
+	bobDb := testChannelStateDB(t, firstBobChannel).GetParentDB()
+	carolDb := testChannelStateDB(t, carolChannel).GetParentDB()
 
 	hopNetwork := newHopNetwork()
 
@@ -1233,8 +1247,8 @@ func newTwoHopNetwork(t testing.TB,
 	aliceChannel, bobChannel *lnwallet.LightningChannel,
 	startingHeight uint32) *twoHopNetwork {
 
-	aliceDb := aliceChannel.State().Db.GetParentDB()
-	bobDb := bobChannel.State().Db.GetParentDB()
+	aliceDb := testChannelStateDB(t, aliceChannel).GetParentDB()
+	bobDb := testChannelStateDB(t, bobChannel).GetParentDB()
 
 	hopNetwork := newHopNetwork()