multi: add SCB restore support for production taproot channels

Add the missing SimpleTaprootFinalVersion case to
chanrestore.openChannelShell() so that SCB backups created for
production taproot channels can be properly restored. Without this,
the channel type bits were not reconstructed during restore, causing
DLP to fail.

Also add integration tests for both confirmed and zero-conf variants
of production taproot channel backup restoration.

Author: Roasbeef

chanrestore.go

@@ -170,6 +170,13 @@ func (c *chanDBRestorer) openChannelShell(backup chanbackup.Single) (
 		chanType |= channeldb.SimpleTaprootFeatureBit
 		chanType |= channeldb.TapscriptRootBit
 
+	case chanbackup.SimpleTaprootFinalVersion:
+		chanType = channeldb.ZeroHtlcTxFeeBit
+		chanType |= channeldb.AnchorOutputsBit
+		chanType |= channeldb.SingleFunderTweaklessBit
+		chanType |= channeldb.SimpleTaprootFeatureBit
+		chanType |= channeldb.TaprootFinalBit
+
 	default:
 		return nil, fmt.Errorf("unknown Single version: %w", err)
 	}

itest/lnd_channel_backup_test.go

@@ -85,6 +85,30 @@ var channelRestoreTestCases = []*lntest.TestCase{
 			)
 		},
 	},
+	{
+		// Restore a channel back up of a confirmed production
+		// taproot channel.
+		Name: "restore simple taproot final",
+		TestFunc: func(ht *lntest.HarnessTest) {
+			runChanRestoreScenarioCommitTypes(
+				ht,
+				lnrpc.CommitmentType_SIMPLE_TAPROOT_FINAL,
+				false,
+			)
+		},
+	},
+	{
+		// Restore a channel back up of an unconfirmed production
+		// taproot channel.
+		Name: "restore simple taproot final zero conf",
+		TestFunc: func(ht *lntest.HarnessTest) {
+			runChanRestoreScenarioCommitTypes(
+				ht,
+				lnrpc.CommitmentType_SIMPLE_TAPROOT_FINAL,
+				true,
+			)
+		},
+	},
 	{
 		Name:     "restore from rpc",
 		TestFunc: testChannelBackupRestoreFromRPC,

lnwallet/channel.go

@@ -922,8 +922,11 @@ func WithAuxResolver(resolver AuxContractResolver) ChannelOpt {
 }
 
 // WithCustomSigningRand is used to provide a custom random source for
-// generating deterministic JIT signing nonces in MuSig2 sessions. This should
-// only be used in tests that need reproducible MuSig2 signatures.
+// generating deterministic JIT signing nonces in MuSig2 sessions.
+//
+// WARNING: This MUST only be used for test vector generation. Setting this in
+// production will produce deterministic nonces, enabling private key extraction
+// via nonce reuse.
 func WithCustomSigningRand(rand io.Reader) ChannelOpt {
 	return func(o *channelOpts) {
 		o.customSigningRand = fn.Some[io.Reader](rand)