ackFlushedDeviceGeneratedOutbound
fun ackFlushedDeviceGeneratedOutbound(deviceId: UInt, flushedPendingHeartbeats: List<ByteArray>, flushedPayload: List<ByteArray>): Boolean
After a successful push of persisted outbound data, removes only flushedPendingHeartbeats and flushedPayload chunk prefixes from disk. Merges that appended after the snapshot remain. Returns false if the file exists but no longer has those exact prefixes (nothing is written).