fix: use updated latest_finalized.slot for justifiability checks

[shariqnaiyer]

leanEthereum/leanSpec#443

The above PR in leanSpec removed the use of the original_finalized_slot snapshot and changed it to using the updated latest_finalized.slot from state. A difference in this implementation within clients leads to some state root mismatches at certain blocks.

I am making this PR so we can get consistency across clients prior to the devnet run in about 6-8 hours. Please feel to request changes, merge or close this PR.

[greptile-apps]

Greptile Summary

This PR aligns the client implementation with leanSpec PR #443 by removing the original_finalized_slot snapshot and having is_valid_vote and try_finalize read state.latest_finalized.slot live. This means that if finalization advances mid-loop during process_attestations, subsequent attestation validity checks and finalization candidate checks operate against the already-updated finalized slot rather than a stale snapshot, producing consistent state roots across clients.

Key changes:

• Removed original_finalized_slot local variable from process_attestations.
• is_valid_vote no longer accepts an original_finalized_slot parameter; it now reads state.latest_finalized.slot directly when calling slot_is_justifiable_after.
• try_finalize no longer accepts an original_finalized_slot parameter; the justifiability guard also reads state.latest_finalized.slot live.
• The behavioral implication is that once finalization advances within the attestation loop, all remaining attestations in the same block see the updated finalized slot — matching the spec semantics introduced in leanSpec PR #443.
• No logic errors were identified; the implementation faithfully mirrors the described spec change. The existing concern around root_to_slot[root] panicking for roots not present in the map (in try_finalize's justifications.retain) is pre-existing and unrelated to this PR.

Confidence Score: 4/5

• This PR is safe to merge; the change is a focused, well-scoped alignment with an upstream spec update and does not introduce new logic errors.
• The diff is small, mechanical, and directly mirrors leanSpec PR #443. The removal of the snapshot simplifies the function signatures and the live reads are consistent throughout process_attestations. No new failure modes are introduced — the only pre-existing concern (potential panic in root_to_slot[root] inside try_finalize) already existed before this PR and is unaffected by the change.
• No files require special attention; the single changed file has a straightforward, low-risk modification.

Important Files Changed

Filename	Overview
crates/blockchain/state_transition/src/lib.rs	Removes original_finalized_slot snapshot; is_valid_vote and try_finalize now read state.latest_finalized.slot live, allowing mid-loop finalization advances to affect subsequent attestation checks — aligning with leanSpec PR #443.

Sequence Diagram

sequenceDiagram
    participant PA as process_attestations
    participant IV as is_valid_vote
    participant TF as try_finalize
    participant S as state

    Note over PA,S: Before PR: original_finalized_slot snapshot taken once
    Note over PA,S: After PR: state.latest_finalized.slot read live each time

    loop For each attestation
        PA->>IV: is_valid_vote(state, source, target)
        IV->>S: read state.latest_finalized.slot (live)
        IV-->>PA: valid / invalid

        alt supermajority reached
            PA->>S: update state.latest_justified
            PA->>TF: try_finalize(state, source, target, ...)
            TF->>S: read state.latest_finalized.slot (live, may be updated)
            alt no justifiable slots between source and target
                TF->>S: state.latest_finalized = source  (advances finalized slot)
                Note over S: latest_finalized.slot updated — affects remaining loop iterations
            else justifiable slots exist
                TF-->>PA: return early (no finalization)
            end
        end
    end

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_{Last reviewed commit: cf2866b}

[MegaRedHand]

Thanks!