[frisitano]

Summary

zkboost v0.12.0's kind: mock zkVMs submit an SSZ MockProof { public_values: List[byte], proof: List[byte] } with random proof bytes (eth-act/zkboost#87). The in-process ERE verifiers reject these, so an EIP-8025 devnet with mock proving cannot run against this branch, and every devnet then needs a GPU prover.

This extends the existing MockProofEngine to accept that format and adds a devnet-only flag to select it. The code names it after the format, not zkboost: any prover may send a MockProof, and zkboost is cited once, as the source of the layout.

• MockProof mirrors zkboost's SSZ container (crates/types/src/execution_proof.rs) with a derived SSZ codec. A layout test pins its encoding to the one zkboost tests.
• MockProofEngine::accepting_mock_proofs(proof_types) accepts a proof of a configured type when it decodes as a MockProof whose public values equal the SSZ-serialized public input of the proof, the same bytes the ERE path compares. It does not check the proof bytes. A proof of an unconfigured type returns UnconfiguredProofType, matching the ERE engine.
• ProofEngineConfig::with_mock_proofs() makes build_engine return that mock engine for the configured proof types. The default is all types; a --proof-engine <path> config narrows them.
• --proof-engine-mock-proofs (requires --proof-engine) enables it and logs a warning.

Prysm optional-proofs has the equivalent --zkvm-mock-proofs. One difference: Prysm also runs a genuine stand-in proof through ERE for each mock proof to keep the verification cost realistic. This PR does not.

Usage (ethereum-package, after ethpandaops/ethereum-package#1492)

participants:
  - el_type: geth
    cl_type: lighthouse
    cl_image: <this branch>
    cl_extra_params:
      - --execution-endpoint=http://zkboost:3000
      - --proof-engine
      - --proof-engine-mock-proofs
additional_services: [zkboost]
zkboost_params:
  image: ghcr.io/eth-act/zkboost/zkboost:0.12.0
  zkvms:
    - kind: mock
      proof_type: ethrex-sp1

Testing

• cargo test -p proof_engine: 10 passed, including the new mock_proof_layout, mock_proofs_verify_their_public_values and exact_proof_data_mock_rejects_mock_proofs.
• cargo clippy -p proof_engine --all-targets --no-deps -- -D warnings: clean. Without --no-deps, clippy 1.95 flags enum_variant_names on SszError in execution_layer, which this PR does not touch.
• Added CLI tests proof_engine_mock_proofs_flag and proof_engine_without_mock_proofs_flag in lighthouse/tests/beacon_node.rs. These are not run locally yet.
• Kurtosis devnets with Prysm optional-proofs and dapplion/lighthouse eip8025: this node accepted every mock proof from the other clients and they accepted every one of its proofs, with none rejected.

Notes for review

• book/src/help_bn.md is not regenerated yet. make cli may flag it.
• The mock lives in test_utils.rs, which is compiled unconditionally. Move it if you'd rather keep devnet code out of that module.