multi: enable closed-channel tombstones on KV-SQL backends

diff --git a/config_builder.go b/config_builder.go
index 394d379..8da8b39 100644
--- a/config_builder.go
+++ b/config_builder.go
@@ -1072,6 +1072,15 @@ func (d *DefaultDatabaseBuilder) BuildDatabase(
 		)
 	}
 
+	// KV-over-SQL backends (sqlite, postgres) opt in to closing channels
+	// via tombstone markers because nested-bucket deletes inside a write
+	// transaction translate into a long-running ON DELETE CASCADE on the
+	// kvdb-on-SQL schema, holding the database write-lock for many seconds
+	// on long-lived channels. bbolt and etcd keep the synchronous one-shot
+	// close path, where nested-bucket deletion is already cheap.
+	tombstoneClosedChans := cfg.DB.Backend == lncfg.SqliteBackend ||
+		cfg.DB.Backend == lncfg.PostgresBackend
+
 	dbOptions := []channeldb.OptionModifier{
 		channeldb.OptionDryRunMigration(cfg.DryRunMigration),
 		channeldb.OptionStoreFinalHtlcResolutions(
@@ -1081,6 +1090,7 @@ func (d *DefaultDatabaseBuilder) BuildDatabase(
 		channeldb.OptionNoRevLogAmtData(cfg.DB.NoRevLogAmtData),
 		channeldb.OptionGcDecayedLog(cfg.DB.NoGcDecayedLog),
 		channeldb.OptionWithDecayedLogDB(dbs.DecayedLogDB),
+		channeldb.OptionTombstoneClosedChannels(tombstoneClosedChans),
 	}
 
 	// Otherwise, we'll open two instances, one for the state we only need
diff --git a/itest/lnd_wipe_fwdpkgs_test.go b/itest/lnd_wipe_fwdpkgs_test.go
index 337331f..d0c6695 100644
--- a/itest/lnd_wipe_fwdpkgs_test.go
+++ b/itest/lnd_wipe_fwdpkgs_test.go
@@ -85,13 +85,28 @@ func testWipeForwardingPackages(ht *lntest.HarnessTest) {
 	// close channel should now become pending force closed channel.
 	pendingAB = ht.AssertChannelPendingForceClose(bob, chanPointAB).Channel
 
-	// Check the forwarding pacakges are deleted.
-	require.Zero(ht, pendingAB.NumForwardingPackages)
-
-	// For Alice, the forwarding packages should have been wiped too.
-	pending := ht.AssertChannelPendingForceClose(alice, chanPointAB)
-	pendingAB = pending.Channel
-	require.Zero(ht, pendingAB.NumForwardingPackages)
+	// On backends that close channels via tombstone markers (sqlite,
+	// postgres), the per-channel forwarding-package bucket is left on
+	// disk by design — the synchronous close path's nested-bucket
+	// delete is exactly what tombstoning avoids. The bytes are reclaimed
+	// by the upcoming native-SQL channel-state migration. The unit-test
+	// suite in channeldb covers the tombstone semantics directly, so
+	// here we just skip the post-close fwd-pkg assertions on those
+	// backends while still exercising the rest of the close flow for
+	// backend symmetry.
+	if !ht.UsesClosedChanTombstones() {
+		require.Zero(ht, pendingAB.NumForwardingPackages)
+
+		// For Alice, the forwarding packages should have been wiped
+		// too.
+		pending := ht.AssertChannelPendingForceClose(alice, chanPointAB)
+		pendingAB = pending.Channel
+		require.Zero(ht, pendingAB.NumForwardingPackages)
+	} else {
+		// Still drive Alice's pending-force-close lookup so the rest
+		// of the test stays backend-symmetric.
+		ht.AssertChannelPendingForceClose(alice, chanPointAB)
+	}
 
 	// Alice should one pending sweep.
 	ht.AssertNumPendingSweeps(alice, 1)
diff --git a/lntest/harness.go b/lntest/harness.go
index 08c62a9..62cbac3 100644
--- a/lntest/harness.go
+++ b/lntest/harness.go
@@ -1444,6 +1444,18 @@ func (h *HarnessTest) IsPostgresBackend() bool {
 	return h.manager.dbBackend == node.BackendPostgres
 }
 
+// UsesClosedChanTombstones reports whether the test harness's database
+// backend closes channels via tombstone markers rather than cascading the
+// nested-bucket delete. This is true on the KV-over-SQL backends (sqlite,
+// postgres) and false on bbolt. Tests that observe forwarding-package or
+// revocation-log deletion immediately after a channel close should consult
+// this predicate; on tombstone backends the bulk state remains on disk
+// until the upcoming native-SQL channel-state migration reclaims it.
+func (h *HarnessTest) UsesClosedChanTombstones() bool {
+	return h.manager.dbBackend == node.BackendSqlite ||
+		h.manager.dbBackend == node.BackendPostgres
+}
+
 // fundCoins attempts to send amt satoshis from the internal mining node to the
 // targeted lightning node. The confirmed boolean indicates whether the
 // transaction that pays to the target should confirm. For neutrino backend,