kona-sp1-proposer: Fill metric gaps blocking alerting

Gaps found while planning alerting for the proposer. Each one either has no metric behind it or has one that can't be alerted on precisely.

## Add

- Deadline-passed counter. `DeadlineStatus::Passed` in `should_skip_proving` only logs at ERROR. That's the moment a game is definitively lost, and it's currently invisible to monitoring.
- `kona_sp1_proposer_up`. op-challenger and op-dispute-mon both export an _up gauge and our down-alerts for those services are built on it.
- Signer ETH balance. The proposer pays `init_bond` per create plus gas on resolve/prove/claim.
- PROVE balance. Only meaningful in network mode since mock has no balance.

Start in the proposer code around should_skip_proving and DeadlineStatus::Passed, then compare the existing _up metrics exported by op-challenger and op-dispute-mon. Define the requested deadline, proposer health, signer ETH balance, and network-mode PROVE balance metrics, and verify that each exposes the conditions needed for precise alerting.

Written by the indexing model from the issue text.