M4 R4.1: pragmatic store-and-forward durable outbox

Adds AVEVA.Historian.Client.StoreForward — a client-side store-and-forward
layer over the historian write surface (AddHistoricalValuesAsync /
SendEventAsync). Producers enqueue writes; the writer persists them and
replays on reconnect so a transient disconnect never drops data. This is the
roadmap's recommended pragmatic outbox, NOT a bit-faithful reimplementation of
AVEVA's native SF cache (that stays deferred) — pure managed, no RE.

- HistorianOutboxEntry / HistorianOutboxEntryKind: buffered-write envelope
- IHistorianOutboxStore + InMemoryHistorianOutboxStore (tests) +
  FileHistorianOutboxStore (crash-durable: atomic temp+move JSON per entry,
  FIFO by filename sequence that resumes past on-disk max, corrupt-file
  quarantine). OutboxJson normalizes event object? properties off JsonElement.
- IHistorianWriteSink + HistorianClientWriteSink (HistorianClient-backed)
- HistorianStoreForwardWriter: enqueue, single-flight FIFO FlushAsync with
  head-of-line blocking, optional MaxDeliveryAttempts dead-lettering,
  DropOldest/Reject overflow policy, background drain loop (retry on reconnect),
  GetStatusAsync snapshot mirroring server SF Pending/Storing/ErrorOccurred.

12 unit tests (no server): durability-across-restart, reconnect-drain, FIFO
order/head-of-line, dead-letter, overflow policies, background auto-drain.
Full suite 293 green. Roadmap R4.1 marked shipped.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01B6mcaT2PjRFKcogzp9UkfC

src/AVEVA.Historian.Client/StoreForward/HistorianClientWriteSink.cs

@@ -0,0 +1,25 @@
+using AVEVA.Historian.Client.Models;
+
+namespace AVEVA.Historian.Client.StoreForward;
+
+/// <summary>
+/// Default <see cref="IHistorianWriteSink"/> that forwards buffered writes through a
+/// <see cref="HistorianClient"/>. Historical values replay via
+/// <see cref="HistorianClient.AddHistoricalValuesAsync"/> and events via
+/// <see cref="HistorianClient.SendEventAsync"/>.
+/// </summary>
+public sealed class HistorianClientWriteSink : IHistorianWriteSink
+{
+    private readonly HistorianClient _client;
+
+    public HistorianClientWriteSink(HistorianClient client)
+    {
+        _client = client ?? throw new ArgumentNullException(nameof(client));
+    }
+
+    public Task<bool> SendHistoricalValuesAsync(string tag, IReadOnlyList<HistorianHistoricalValue> values, CancellationToken cancellationToken) =>
+        _client.AddHistoricalValuesAsync(tag, values, cancellationToken);
+
+    public Task<bool> SendEventAsync(HistorianEvent historianEvent, CancellationToken cancellationToken) =>
+        _client.SendEventAsync(historianEvent, cancellationToken);
+}