chore(metrics): emit drop-stage counter for liquidator-bytecode-missing

[obchain]

Problem

On a misconfigured fork where the liquidator address has no bytecode, every opportunity drops at the simulation stage with a generic record_opportunity_dropped_reason(SIM_REVERT). There is no signal in the dashboard separating "the liquidation logic reverted" from "the contract isn't deployed at the address we're calling". Operators staring at a quiet Grafana cannot tell which problem they have.

Evidence

crates/charon-cli/src/main.rs:2016-2022:

if let Err(err) = gate.encode_and_simulate(&opp, &params).await {
    charon_metrics::record_simulation(chain, sim_result::REVERT);
    charon_metrics::record_opportunity_dropped(chain, drop_stage::SIMULATION);
    charon_metrics::record_opportunity_dropped_reason(chain, drop_reason::SIM_REVERT);
    ...
}

Proposed fix

Detect the empty-bytecode case once at startup (paired bytecode-check issue) and refuse to start; and add a drop_reason::LIQUIDATOR_NOT_DEPLOYED so a contract that gets self-destructed mid-run still surfaces a distinct counter. Update Grafana's "Executor — opportunities queued vs dropped" panel to break out drops by reason as well as stage.

Acceptance criteria

• New drop_reason constant added in charon-metrics.
• CLI emits the reason when simulate returns the well-known "no code at address" revert payload.
• Grafana panel updated.

Severity

medium (low impact once startup bytecode check lands; covers runtime self-destruct edge case)