WriteAheadLog (6.x): make a committed write durable across a graceful restart with a batched FlushInterval #284

DotNext.Net.Cluster 6.3.0, .NET 10. RaftCluster (TCP) with a hand-assigned WriteAheadLog as AuditTrail over a SimpleStateMachine, one cluster per pod. Membership and restart leadership are already solved with a persistent configuration storage (per #207); this question is purely about WAL durability.

I expose a synchronous storage API, so a write must be durable before it returns, or at minimum survive a graceful (SIGTERM) restart. Completion is detected with a waiter completed inside SimpleStateMachine.ApplyAsync: I await ReplicateAsync(...) then await the waiter, so a write returns once the entry is applied.

• With FlushInterval = TimeSpan.Zero (flush on every commit in the background) this is reliable: a node restarted from the same WAL recovers everything via InitializeAsync. But it fsyncs on every commit.
• With a positive FlushInterval (to batch flushes for throughput), a graceful shutdown loses the un-snapshotted tail. A single committed entry written then immediately followed by DisposeAsync is not recovered after a restart, even when I call FlushAsync() before disposing the cluster and the log. A longer run (e.g. 200 writes) is recovered, because the periodic flusher and snapshots cover it.

Questions:

1. Is there a supported way to make a committed write durable on return — awaiting the flush, not just the apply — so the caller knows it is on stable storage?
2. With a hand-assigned WriteAheadLog, what is the correct flush-on-graceful-shutdown sequence so a positive FlushInterval does not lose the most recent committed-but-unflushed entries? Is FlushAsync() the right call, and where must it run relative to RaftCluster.StopAsync() and disposal?
3. Or is FlushInterval = TimeSpan.Zero the only mode that guarantees a committed entry survives a restart, with any positive interval being best-effort (up to one interval lost on a stop)?

Goal: one cluster per pod that can batch flushes for throughput yet never lose a committed write across a graceful restart. Happy to provide a minimal repro.