Fix darkside v5 txid computation and tree state nil dereference

darksideSetTxID applied SHA256d to all transactions including v5, producing
incorrect txids. Now that ParseFromSlice computes v5 txids via ZIP 244,
skip the SHA256d override for v5 transactions. (fixes #456)

DarksideRemoveTreeState crashed with a nil pointer dereference when
removing a tree state at a non-existent height or hash. Add nil checks
to return an error instead of panicking. (fixes #466)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: zmanian

common/darkside.go

@@ -112,10 +112,11 @@ type darksideProtocolSubtreeRoots struct {
 var DarksideEnabled bool
 
 func darksideSetTxID(tx *parser.Transaction) {
-	// SHA256d
-	// This is correct for V4 transactions, but not for V5, but in this test
-	// environment, it's harmless (the incorrect txid calculation can't be
-	// detected). This will be fixed when lightwalletd calculates txids correctly .
+	if tx.IsV5() {
+		// V5 txids are computed via ZIP 244 during ParseFromSlice.
+		return
+	}
+	// SHA256d (correct for pre-v5 transactions).
 	digest := sha256.Sum256(tx.Bytes())
 	digest = sha256.Sum256(digest[:])
 	tx.SetTxID(digest)
@@ -935,11 +936,17 @@ func DarksideRemoveTreeState(arg *walletrpc.BlockID) error {
 	}
 	if arg.Height > 0 {
 		treestate := state.stagedTreeStates[arg.Height]
+		if treestate == nil {
+			return fmt.Errorf("no tree state at height %d", arg.Height)
+		}
 		delete(state.stagedTreeStatesByHash, treestate.Hash)
 		delete(state.stagedTreeStates, treestate.Height)
 	} else {
 		h := hex.EncodeToString(arg.Hash)
 		treestate := state.stagedTreeStatesByHash[h]
+		if treestate == nil {
+			return fmt.Errorf("no tree state for hash %s", h)
+		}
 		delete(state.stagedTreeStatesByHash, treestate.Hash)
 		delete(state.stagedTreeStates, treestate.Height)
 	}

parser/transaction.go

@@ -646,6 +646,11 @@ func (tx *Transaction) isZip225V5() bool {
 		tx.version == ZIP225_TX_VERSION
 }
 
+// IsV5 reports whether the transaction uses the ZIP 225 v5 format.
+func (tx *Transaction) IsV5() bool {
+	return tx.isZip225V5()
+}
+
 func (tx *Transaction) isGroth16Proof() bool {
 	// Sapling changed the joinSplit proof from PHGR (BCTV14) to Groth16;
 	// this applies also to versions beyond Sapling.