Track SuperAuthority finalized L2 head in metrics

## Summary

Add a dedicated metric for the SuperAuthority finalized L2 head/block.

PR #20872 intentionally stops using `EngineController.SetFinalizedHead` when materializing a SuperAuthority finalized ref, because `SetFinalizedHead` mutates `localFinalizedHead` / `deprecatedFinalizedHead`. That mutation is the wrong state model for SuperAuthority finality, but it also means the existing `l2_finalized` ref metric is no longer updated from the SuperAuthority path.

## Context

Review thread: https://github.com/ethereum-optimism/optimism/pull/20872#discussion_r3268284287

Current metrics I found:

- `op-node` records generic `l2_finalized` via `SetFinalizedHead`.
- `op-supervisor` records local/cross safe and unsafe refs, but not finalized refs.
- `op-supernode` records interop verification counters/timestamps, but not the SuperAuthority finalized L2 block/ref itself.

## Proposed direction

Add a dedicated SuperAuthority finality ref metric, for example one of:

- an `op-node` ref metric such as `l2_super_authority_finalized`, recorded when `FinalizedHead()` successfully resolves the SuperAuthority finalized block; or
- a SuperAuthority/supernode-owned metric that records the finalized L2 block/hash/number per chain when the authority computes it.

The important property is that dashboards can track SuperAuthority finality progress without overloading `l2_finalized`, which is tied to local engine/controller finalized state.

## Acceptance criteria

- SuperAuthority finalized L2 progress is exposed as a scraped metric.
- The metric is distinct from local finalized state.
- The metric includes enough information to track finality lag/progress, ideally block number and hash, and chain ID where applicable.
- Tests cover that the metric is updated when SuperAuthority finalized output advances.

Start by tracing op-node's SetFinalizedHead and FinalizedHead paths, then compare the finalized-ref handling in op-supervisor and op-supernode. Decide where the dedicated SuperAuthority metric belongs and inspect the existing ref metrics there. Done means a distinct scraped metric exposes advancing SuperAuthority finality with block details and tests verify its updates.

Written by the indexing model from the issue text.