apollo_consensus_orchestrator: use HashSet instead of IndexSet for duplicate-tx tracking

[gkaempfer]

Summary

Follow-up performance review of #14840, scoped to the duplicate-transaction-hash tracking it added to the proposal validation hot path in crates/apollo_consensus_orchestrator/src/validate_proposal.rs.

#14840 introduced seen_tx_hashes: IndexSet<TransactionHash> in validate_proposal, populated once per transaction in every proposal part received during consensus proposal validation (handle_proposal_part) — a genuine per-transaction, per-block hot path.

seen_tx_hashes is only ever inserted into and queried for membership (!seen_tx_hashes.insert(tx_hash)); it is never iterated or indexed anywhere in the file (verified via grep -n "seen_tx_hashes" -r crates/apollo_consensus_orchestrator/src/). IndexSet (from indexmap) exists specifically to provide insertion-order iteration, at the cost of an auxiliary dense-vector layer on top of the hash table — overhead this call site never benefits from. Swapping to std::collections::HashSet removes that bookkeeping from every transaction insertion, with a smaller memory footprint, while leaving behavior identical (insert returns bool = "was newly inserted" on both types; TransactionHash: Hash + Eq satisfies both).

The removed use indexmap::IndexSet; was the only use of IndexSet in this file; indexmap remains a dependency of the crate for unrelated usage elsewhere (cende/central_objects.rs).

Test plan

• cargo build -p apollo_consensus_orchestrator — clean
• SEED=0 cargo test -p apollo_consensus_orchestrator — 177 passed, 0 failed, including duplicate_transaction_hash_in_proposal_is_rejected_before_batcher, which exercises this exact code path
• Reviewed by an independent Opus pass: confirmed the set is never iterated elsewhere, the substitution is behavior-preserving, and the import removal breaks nothing — no blocking or suggestion findings

🤖 Generated with Claude Code

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Generated by Claude Code

[cursor]

PR Summary

Low Risk
Behavior-preserving type swap on membership-only duplicate detection; consensus validation logic is unchanged aside from data structure choice.

Overview
Swaps IndexSet<TransactionHash> for std::collections::HashSet<TransactionHash> when tracking transaction hashes during consensus proposal validation in validate_proposal.rs.

Duplicate detection still uses !seen_tx_hashes.insert(tx_hash) in handle_proposal_part; insertion order was never used, so dropping indexmap’s extra ordering layer should cut per-tx overhead on that hot path without changing rejection behavior.

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[reviewable-StarkWare]

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