feat(parsigex): add eth2-based partial signature verifier

[emlautarom1-agent]

Summary

Implements eth2 signature verification for inbound partial signatures in parsigex, so a peer's partial signatures are cryptographically validated before they are accepted into the partial-signature database.

What it implements

• parsigex::new_eth2_verifier(eth2_cl, pub_shares_by_key) -> Verifier — for each inbound partial signature received for a validator, looks up the sender's public share by the signature's share index and verifies the eth2 BLS signature via pluto_core::verify_eth2_signed_data. Partial signatures with an unknown validator public key or an unknown share index are rejected.
• Adds pluto-eth2api and promotes pluto-crypto to runtime dependencies of parsigex, to name the beacon node client and the public-share type.

Ports Charon's core/parsigex/parsigex.go NewEth2Verifier. Part of #402.

Testing

Accept/reject unit tests using a mocked beacon node and threshold BLS shares: a valid partial signature against the correct share verifies, while a wrong share, an unknown public key, and a missing share index are each rejected.

[emlautarom1]

@claude

[claude]

Claude finished @emlautarom1's task in 1m 55s —— View job

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Code Review — eth2-based partial signature verifier

• Read the PR diff and changed files
• Compare against Charon NewEth2Verifier
• Review correctness, error handling, tests
• Post review feedback

Overall this is a clean, faithful port of Charon's core/parsigex/parsigex.go NewEth2Verifier. The check ordering (unknown pubkey → invalid share index → eth2 type assertion → BLS verification) matches the Go source exactly, the type mapping is sound (Index = u64 and ParSignedData::share_idx: u64 line up with Go's map[int]), and the error model is a genuine improvement over the original Other(String) placeholder. Nice work. A few notes below — all minor.

Correctness — looks good ✅

• crates/parsigex/src/behaviour.rs:71-89 mirrors Charon 1:1. The as_eth2_signed_data upcast correctly stands in for Go's data.(core.Eth2SignedData) type assertion, and the comment explaining that mapping is appreciated.
• Dropping the fallible constructor return (Go returns (func, error) but never errors) is the right call.
• Arc-cloning eth2_cl and pub_shares_by_key per invocation is correct and cheap.

Suggestions (non-blocking)

1. Missing test for the InvalidSignedDataFamily path. The tests cover accept / wrong-share / unknown-pubkey / missing-share-index, but not the case where signed_data is not an eth2 signed-data variant (e.g. a raw Signature). Since as_eth2_signed_data is the one branch unique to this verifier that isn't exercised, a small test passing a non-eth2 ParSignedData and asserting VerifyError::InvalidSignedDataFamily would close the matrix. Fix this →

2. Duty embedded in VerifyError::InvalidSignature. This faithfully reproduces Charon's z.Str("duty", duty.String()) log context, and reads well. Just note the only current consumer (handler.rs:265) collapses it via e.to_string() into Failure::InvalidPartialSignature(String), so the structured duty/source fields aren't accessed programmatically yet — the value is purely in the Display output today. Fine as-is, but worth being aware of if anyone expects to match on it.

Nits

• crates/parsigex/Cargo.toml: promoting pluto-crypto to a runtime dep and adding pluto-eth2api (runtime) / pluto-eth2util (dev) is correct and matches usage.
• Doc comment on new_eth2_verifier is thorough and the intra-doc links resolve.

I could not run cargo test/clippy in this environment (the commands require approval here), so I reviewed by inspection — please rely on CI for the build/lint gates.
• emlautarom1/parsigex-eth2-verifier