feat: bind BLS pop to chainId and registrantAddress

[shahin-hq]

Closes: https://app.clickup.com/t/2570579/86e3gp5ck

Summary

Updates BLS proof of possession to match Mainsail's new format (ArkEcosystem/mainsail#1506). Proofs are now tied to a chain and a registering address, so a key/proof pair can't be copied from the mempool or from another network and front-run.

• DST is now MAINSAIL_BLS_POP_BLS12381G2_XMD:SHA-256_SSWU_RO_POP_
• The message is abi.encodePacked(chainId, registrantAddress, pk), with chainId taken from the configured network
• validatorPassphrase() is renamed to validatorProof($passphrase, $registrantAddress), which is a breaking change
• buildProofOfPossession(), fromMnemonic() and the TransactionEncoder validator methods now take $registrantAddress. It must be the address that signs the transaction.
• Fixtures were regenerated, and the tests reproduce Mainsail's pinned vector byte for byte

Checklist

• Documentation (if necessary)
• Tests (if necessary)
• Ready to be merged