Offline-first message buffer with sequence-number-ordered replay for ManagedClient

## Problem Statement

When an MQTT client reconnects after a connectivity
outage, the .NET MQTTnet library provides no built-in
mechanism to:

1. **Buffer messages locally** during the outage without
blocking the application
2. **Persist buffered messages to disk** so they survive
process restarts
3. **Replay messages in guaranteed chronological order**
when connectivity is restored

The MQTTnet wiki explicitly acknowledges this gap:
> "It is recommended to use the regular client and
> doing the reconnect stuff etc. via your own code."

However, most developers implementing their own reconnect
logic make a critical mistake: they replay in FIFO queue
order or timestamp order. During connectivity outages,
device clocks drift from NTP synchronization — meaning
timestamps become unreliable for ordering. Messages
replayed in timestamp order arrive at downstream
consumers out of sequence after clock correction.

## Real-World Impact

In enterprise IoT deployments across geographically
distributed locations — financial services, healthcare,
industrial safety — this silent data loss has direct
compliance and operational consequences.

We observed message loss rates of 0.3%–4.7% across
branch locations before solving this problem.

## The Key Insight — Sequence Numbers Not Timestamps

The correct solution assigns a monotonically increasing
sequence number at write time — immune to clock drift —
and uses that for ordering rather than device timestamps.

## Reference Implementation

I have built an open source .NET library addressing
this gap:

GitHub: https://github.com/rambudithe/EdgeSyncBuffer
NuGet: https://www.nuget.org/packages/EdgeSync.Buffer