[scanner] Mempool monitor: pending Venus oracle-update txs + pre-computed impacted positions

[obchain]

Context

When a price oracle update is pending in the mempool, its effect on health factors is already knowable — no need to wait for the block to land. Pre-computing liquidation txs against the incoming price gives us a head start on every competing bot that waits for confirmation.

Scope

• charon-scanner/src/mempool.rs with a MempoolMonitor task
• subscribe_pending_transactions() via alloy pub-sub
• Detect txs whose to address matches a known oracle (Venus Oracle, Chainlink, etc.)
• Decode the new price from calldata
• Simulate: which tracked positions would cross health_factor < 1.0 at the new price?
• Push pre-computed LiquidationOpportunity into the executor queue, flagged as "pre-computed"
• When the block lands, fire immediately (zero compute delay)

Acceptance criteria

• Mempool subscription runs as a spawned task
• Oracle update transactions are detected and logged (no false positives on unrelated txs)
• Price-impact simulation produces the same Position.health_factor as a fresh scan would post-confirmation
• Pre-computed opportunities are tagged in logs and can be distinguished from scan-driven ones
• Stable for 10+ minutes without memory growth

References

• PRD section 6b (Mempool Monitoring & Pre-computation)
• PRD Diagram 06 (MEV Defense Pipeline)

[obchain]

Superseded — mempool monitor shipped in feat/21-mempool-monitor.