feat: bind validator PoP to chainId and registrantAddress

[shahin-hq]

Summary

Closes: https://app.clickup.com/t/2570579/86e3gp5cw

Binds the validator BLS proof of possession (PoP) to the chainId and the registrant address, as Mainsail now requires. Without this, a PoP could be replayed by another sender or on another chain.

• BuildValidatorRegistration, BuildValidatorUpdate, EncodeValidatorRegistrationData, EncodeValidatorUpdateData, FromMnemonic and BuildProofOfPossession take a new registrantAddress parameter.
• The PoP now signs abi.encodePacked(uint256 chainId, address registrant, bytes pk) under the MAINSAIL_BLS_POP_BLS12381G2_XMD:SHA-256_SSWU_RO_POP_ domain tag. chainId comes from the configured network.
• The registrant address is validated, including the EIP-55 checksum. An invalid address returns ErrInvalidProofOfPossession.
• Adds tests for Mainsail's reference vector (chainId 10_000), the chainId and address binding, and invalid addresses. The validator transaction fixtures and the multi-language BLS fixture are regenerated.

Checklist

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