[Usage]: Mooncake HA: Primary-side OpLog writing not wired — by design or pending work?

### Describe your usage question

With `enable_ha` enabled, the Leader Election and Standby OpLog replay pipeline are both implemented, but mutation RPCs (`PutEnd`, `Remove`, etc.) in `WrappedMasterService` delegate directly to `master_service_` without calling `OpLogManager::Append()` / `AppendAndPersist()`.

Specifically:

- `OpLogManager` is only instantiated in its own implementation and test files — `MasterService` / `WrappedMasterService` have no `oplog_manager_` member
- The only production usage of `enable_ha_` is in `ResolveSnapshotSequenceId()` (a read-only snapshot boundary query)
- Standby-side components (`HotStandbyService`, `OpLogApplier`, `OpLogReplicator`) are implemented but receive no entries since Primary never writes OpLog

This means after failover, a promoted Standby has empty or stale metadata and cannot serve traffic.

Is this intentional (e.g., currently validating election flow only), or is the integration still pending? If the latter, are there any plans or in-progress PRs for this?

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