M4 R4.4: client-side multi-historian redundancy

Adds AVEVA.Historian.Client.Redundancy — HistorianRedundantClient orchestrates
N single-historian members (IHistorianMember; default HistorianClientMember
over HistorianClient) as one logical client. Pure client-side, no server-side
redundancy protocol, no RE.

- Reads fail over to the next member in priority order. Streaming reads only
  fail over BEFORE the first row is observed; a mid-stream failure propagates
  (failing over mid-stream would risk duplicated/skipped rows).
- Writes fan out: WriteFanout AllMembers | PreferredOnly, with All | Any ack
  policy, returning a per-member HistorianRedundantWriteResult.
- Per-member health: FailureThreshold demotes a failing member out of the
  preferred pool; a background watchdog (PeriodicTimer) + CheckHealthAsync
  re-probe and restore recovered members. GetStatus() snapshot + ActiveMember.
- Composes with R4.1: back a member's writes with a HistorianStoreForwardWriter
  so a down member buffers and replays on recovery — the pragmatic client-side
  equivalent of native ReSyncTags.

14 unit tests (no server): failover order, mid-stream no-failover, all-fail
aggregation, probe-any-up, fan-out ack policies, PreferredOnly, soft reject,
health demotion + CheckHealthAsync restore, watchdog recovery. Full suite 307
green. Roadmap R4.4 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/Redundancy/HistorianAllMembersFailedException.cs

@@ -0,0 +1,18 @@
+namespace AVEVA.Historian.Client.Redundancy;
+
+/// <summary>
+/// Thrown by a <see cref="HistorianRedundantClient"/> read when every member failed the operation.
+/// The per-member failures are aggregated in <see cref="Exception.InnerException"/> (an
+/// <see cref="AggregateException"/>).
+/// </summary>
+public sealed class HistorianAllMembersFailedException : Exception
+{
+    public HistorianAllMembersFailedException(string operation, IReadOnlyList<Exception> failures)
+        : base($"All historian members failed the '{operation}' operation.", new AggregateException(failures))
+    {
+        Operation = operation;
+    }
+
+    /// <summary>The orchestrated operation that failed across all members.</summary>
+    public string Operation { get; }
+}