feat(mempool-backrun): Phase 1 Part A — Rust validate_backrun revm sim + gRPC publish

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Context

Mempool path today (PR #118 + follow-ups on develop) emits analytical arb candidates and logs them — never publishes to the executor. To make these candidates executable, we need a revm-based validator that confirms the victim + our arb tx land successfully in the same block, then publish a ValidatedArb on the existing gRPC stream so the Go executor can build a bundle.

This issue ships the Rust half of the public-mempool backrun execution path.

Scope

1. crates/simulator/src/mempool_backrun.rs (new)

• validate_backrun(victim_tx, our_calldata) -> SimResult
  • Forks at victim.block_number - 1 via existing crates/simulator::fork::EvmFork
  • Applies victim tx via evm.transact_commit()
  • Applies our arb tx via evm.transact_commit()
  • Returns SimResult { victim_status, arb_status, gross_profit_wei, gas_used, post_state_reserves[] }
• Reject reasons (counted via metric, not returned as Err): victim_reverts, arb_reverts, negative_after_gas, slippage_too_high, victim_not_found_in_state
• Hard 20 ms per-sim timeout
• Concurrent sim semaphore (default 8 in-flight) — env override AETHER_MEMPOOL_SIM_CONCURRENCY

2. crates/grpc-server/src/mempool_pipeline.rs (modify)

• After analytical try_post_state_scan produces a candidate:
  • Build executor calldata via existing crates/simulator::calldata::encode_execute_arb (already used on block-driven path)
  • Call validate_backrun(victim, calldata)
  • On accept: tag ArbOpportunity.source = MempoolBackrun, publish via existing ArbService::SubmitArb broadcast channel
  • On reject: emit aether_mempool_backrun_rejected_total{reason}
• Reject after MEMPOOL_VICTIM_FRESHNESS_MS (default 500 ms) since seen_at to avoid wasting sim on stale victims

3. proto/aether.proto (modify)

• Add enum ArbSource { BlockDriven = 0; MempoolBackrun = 1; } to ArbOpportunity
• Add victim_tx_hash: bytes field (empty for block-driven)
• Add target_block: uint64 field
• Regenerate Rust + Go bindings

4. Metrics

• aether_mempool_backrun_validation_latency_ms histogram (label result=accept|reject)
• aether_mempool_backrun_validated_total{profit_bucket} counter
• aether_mempool_backrun_rejected_total{reason} counter
• aether_mempool_backrun_sim_concurrent gauge

5. Tests

• Unit test for validate_backrun happy path (synthetic V2 victim + arb, revm in-memory)
• Unit test each reject reason
• Integration test that proto change does not break block-driven publish

Acceptance criteria

• validate_backrun accepts V2 + V3 + Curve + Balancer victims (matches predict_post_state coverage)
• Mempool path publishes ValidatedArb via existing ArbService::SubmitArb when sim accepts
• ArbOpportunity.source = MempoolBackrun for mempool-path arbs; BlockDriven unchanged
• Concurrent sim semaphore enforced — never more than AETHER_MEMPOOL_SIM_CONCURRENCY in flight
• 20 ms per-sim timeout enforced; timeout counted as reject:sim_timeout
• cargo build --workspace --release clean
• cargo clippy --workspace --all-targets --release -- -D warnings clean
• cargo test --workspace --release — all pass incl. new tests
• No regression on block-driven path (block-driven arbs still publish + execute)

Out of scope

• Go executor changes — separate issue
• Risk gates + shadow rollout — separate issue
• MEV-Share mev_sendBundle envelope (Phase 2)
• revm sim 5ms → 1ms perf work (Phase 3)

Risk

• Proto change must be backward-compatible — new fields with default values, no removed fields
• revm fork at every mempool tx is expensive — concurrency cap + freshness gate keep p99 under budget
• Sim timeout fail-closed (reject) not fail-open (accept) — never publish unsim'd

Depends on

• PR feat(mempool): live tracking — subscribe + decode + analytical V2 post-state sim #118 ([E5] Live mempool tracking — Phase 0/1 testing scaffold (Alchemy WS + MEV-Share SSE + pending-tx decoder + post-state sim) #117 — mempool scaffold) — merged ✅

Blocks

• feat(mempool-backrun): Phase 1 Part B — Go executor mempool bundle builder + fan-out #139 (Part B) — Go executor mempool bundle builder
• feat(mempool-backrun): Phase 1 Part C — risk gates + shadow rollout + live validation #140 (Part C) — risk gates + shadow rollout + live validation