contractcourt: update breach arbiter to support taproot chans

In this commit, we update the breach arb to support taproot channels. We
utilize the new taproot briefcase space to store both control blocks,
and also the first+second level scripts for the set of HTLCs.

lnwallet/channel.go

@@ -2319,6 +2319,11 @@ type HtlcRetribution struct {
 	// update the SignDesc above accordingly to sweep properly.
 	SecondLevelWitnessScript []byte
 
+	// SecondLevelTapTweak is the tap tweak value needed to spend the
+	// second level output in case the breaching party attempts to publish
+	// it.
+	SecondLevelTapTweak [32]byte
+
 	// IsIncoming is a boolean flag that indicates whether or not this
 	// HTLC was accepted from the counterparty. A false value indicates that
 	// this HTLC was offered by us. This flag is used determine the exact
@@ -2634,6 +2639,16 @@ func createHtlcRetribution(chanState *channeldb.OpenChannel,
 		signDesc.TapTweak = tapscriptRoot[:]
 	}
 
+	// If this is a taproot output, we'll also need to obtain the second
+	// level tap tweak as well.
+	var secondLevelTapTweak [32]byte
+	if chanState.ChanType.IsTaproot() {
+		tapscriptRoot := secondLevelScript.ScriptTree.RootNode.TapHash()
+		copy(
+			secondLevelTapTweak[:], tapscriptRoot[:],
+		)
+	}
+
 	return HtlcRetribution{
 		SignDesc: signDesc,
 		OutPoint: wire.OutPoint{