CVE-2026-34068: nimiq-transaction: UpdateValidator transactions allows voting key change without proof-of-knowledge

The staking contract accepts UpdateValidator transactions that set new_voting_key=Some(...) while omitting new_proof_of_knowledge. this skips the proof-of-knowledge requirement that is needed to prevent BLS rogue-key attacks when public keys are aggregated.

Because tendermint macro block justification verification aggregates validator voting keys and verifies a single aggregated BLS signature against that aggregate public key, a rogue-key voting key in the validator set can allow an attacker to forge a quorum-looking justification while only producing a single signature.

While the impact is critical, the exploitability is low: The voting keys are fixed for the epoch, so the attacker would need to know the next epoch validator set (chosen through VRF), which is unlikely.