Sign SVM Transactions

import software.sava.core.tx.Transaction;
// ... assemble Transaction tx ...
byte[] messageBytes = tx.serializedMessage();  // not tx.serialize() — that includes empty sig slots

import xyz.dynamic.waas.svm.opts.SignTransactionOpts;

String signatureHex = client.signTransaction(SignTransactionOpts.builder()
    .messageBytes(messageBytes)
    .walletProperties(walletProperties)
    .externalServerKeyShares(externalServerKeyShares)
    .build()
).join();

System.out.println("Signature: " + signatureHex);

import java.nio.ByteBuffer;
import software.sava.core.encoding.CompactU16Encoding;
import org.web3j.utils.Numeric;

byte[] sigBytes = Numeric.hexStringToByteArray("0x" + signatureHex);   // 64 bytes

ByteBuffer wire = ByteBuffer.allocate(
    CompactU16Encoding.encodedLength(1) + 64 + messageBytes.length
);
CompactU16Encoding.encode(wire, 1);    // signature count
wire.put(sigBytes);
wire.put(messageBytes);

byte[] signedTxWire = new byte[wire.position()];
wire.flip(); wire.get(signedTxWire);