spec(eip8025): contain invalid proof gossip

[exocognosis]

Summary

EIP-8025 allows any active validator to sign an execution proof. Existing gossip validation rejects invalid proofs and can penalize the forwarding peer, but a malicious validator can rotate inexpensive peer identities while continuing to sign proofs that require expensive proof-engine verification.

This change adds authenticated validator-level containment for unsolicited execution-proof gossip without reintroducing proof re-signing.

Proposed solution

• Ignore future execution_proof gossip signed by a validator that has already authenticated an invalid execution proof.
• Record the validator only after its BLS signature passes and a later rejection condition fails. Messages with invalid signatures cannot frame another validator.
• Retain the invalid-signer record across restarts while the validator remains active.
• Limit signer suppression to unsolicited gossip.
• Continue validating proofs returned by locally initiated ExecutionProofsByRange and ExecutionProofsByRoot requests.
• Keep valid requested proofs eligible for local use and request and response serving even when their signer is suppressed from gossip.
• Require honest provers to verify completed proofs before signing and broadcasting them.

Rationale

Peer scoring remains useful because a gossip rejection penalizes the peer that forwarded the invalid message. It is not sufficient by itself when peer identities can be rotated independently of validator keys.

The authenticated signer record makes the validator key the durable accountability anchor. Each client performs at most one expensive unsolicited invalid-proof verification per validator before suppressing later gossip from that signer.

Applying suppression only to gossip addresses the selective-availability concern raised in the issue. A node that ignored later gossip from a signer can still recover a valid proof through the existing bounded request and response protocols. This removes the need for validators to re-sign every valid proof and avoids the resulting protocol complexity and network amplification.

The mechanism is transitional and does not alter proof containers, signatures, topic encodings, or request limits.

Validation

• make lint
• make test fork=eip8025
  • 1,365 passed
  • 6,431 skipped

Fixes #5145.