[E5] Mempool tap — wire MEV-Share into engine + unified candidate stream + prod filter

[0xfandom]

Follow-up to #117. The Phase 0/1 scaffold (Alchemy WS subscribe, router decoder, MEV-Share SSE monitor, V2 post-state sim) is in place on feat/mempool-tracking-scaffold. This issue tracks the remaining work to make the mempool tap a real trigger on the engine hot path — not just a monitor.

Done already (on branch)

• public mempool subscribe (crates/ingestion/src/mempool.rs)
• router decoder for V2 / Sushi / Curve / V3 (crates/pools/src/router_decoder.rs)
• MEV-Share SSE monitor (cmd/monitor/mev_share.go) — monitor binary only
• V2 analytical post-state sim (crates/grpc-server/src/mempool_pipeline.rs)
• backpressure metering via aether_pending_pipeline_lagged_total
• 60s reconnect on MEV-Share read silence
• O(1) pool lookup via cached PairIndex

To do

• route MEV-Share matched txns from cmd/monitor/mev_share.go into the engine trigger path (not just monitor — same path public mempool already uses)
• unified candidate stream with source={"public","mev_share"} label propagated through detection → arb publish
• production filter rules: drop pending txns whose to is not in pool_registry, drop unknown selectors before decoder is called
• arbs.source column (migration 0003_add_source_to_arbs.sql) so DB rows carry the flow origin
• metric aether_mempool_candidates_total{source} + aether_mempool_filtered_total{reason}

Out of scope (separate issue)

• Multi-provider mempool fanout (only Alchemy WS today, see [E5] Live mempool tracking — Phase 0/1 testing scaffold (Alchemy WS + MEV-Share SSE + pending-tx decoder + post-state sim) #117 caveat)
• Private order flow sources beyond MEV-Share (Flashbots Protect, etc.)

Done when

• 30-min live shadow run shows arbs rows with both source=public and source=mev_share
• filter drop rate matches expectation (>99% of public mempool txns rejected by to-not-in-registry filter)
• no regression in existing aether_pending_pipeline_lagged_total (queue not bottlenecking)

Branch

feat/mempool-tracking-scaffold — currently 13 commits ahead of develop, rebased clean.