v0.2.1

Fixed

• Truncated OP_PUSHDATA1/2/4 scripts now raise ArgumentError instead of crashing with TypeError
• Transaction#to_beef uses merge_bump to correctly handle multiple ancestors at the same block height
• PrivateKey#derive_child uses BN.mod_add instead of Integer roundtrip for modular addition
• Fixed txid byte-order documentation (display order, not internal order)

Testing

• FORKID enforcement spec verifying interpreter rejects signatures without SIGHASH_FORKID
• ExtendedKey fingerprint chain integrity across 3-generation derivation
• Mnemonic entropy round-trip across all 5 valid entropy lengths
• BEEF spec for multiple ancestors at the same block height