STAKE transactions:
- Add buildStakeTransaction() for creating stake transactions
- Add createStakeTransaction() wallet API for staking SAL/SAL1
- Extend serializeTxPrefix() with Salvium-specific fields (txType,
amount_burnt, asset types, return address, protocol_tx_data)
- Support STAKE_LOCK_PERIOD from network config (21600 mainnet, 20 testnet)
CLSAG fixes:
- Fix challenge hash to include full ring data matching C++ implementation
- Update both signing and verification to use consistent hash format
Matt Hess