[scanner] Block listener: subscribe_blocks + structured per-block logging

[obchain]

Context

The bot's heartbeat. Subscribes to newHeads over WebSocket and routes each new block into the scanning pipeline. Structured logging makes it grep-able (chain=bsc block=N) and Prometheus-ready for later.

Scope

• charon-scanner/src/listener.rs with a BlockListener task
• subscribe_blocks() via alloy's pub-sub provider
• Emits events into an mpsc channel consumed by the rest of the pipeline
• Every received block logs structured fields: chain, block, timestamp
• Code path is written for a single chain today but shaped so tokio::select! across multiple chains is a drop-in later

Acceptance criteria

• Listener runs as a spawned tokio task without blocking main
• Every new BSC block produces one structured log line
• Logs are readable at INFO level, verbose at RUST_LOG=debug
• Listener survives 5+ continuous minutes without crashing
• Graceful error handling — reconnect attempt on WebSocket drop

References

• PRD section 5c (Multi-Chain Listener Orchestration)

[obchain]

Superseded — BlockListener shipped in feat/07-block-listener.