feat(historian): backend registry, ServerHistorian:Backend, Host wiring via provider

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Historian/ContinuousHistorizationMetrics.cs

@@ -37,6 +37,11 @@ public static class ContinuousHistorizationMetrics
             ObserveDropped,
             unit: "{entry}",
             description: "Lifetime entries the continuous-historization outbox dropped on capacity overflow.");
+        OtOpcUaTelemetry.Meter.CreateObservableGauge(
+            "otopcua.historian.outbox.unreadable",
+            () => _outbox?.UnreadableCount ?? 0L,
+            unit: "{entry}",
+            description: "Lifetime undecodable records the continuous-historization outbox skipped.");
     }
 
     /// <summary>Binds the process outbox the gauges observe. Called once by the Host when continuous

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Historian/ServerHistorianOptions.cs

@@ -35,6 +35,20 @@ public sealed class ServerHistorianOptions
     ///     The scheme selects the transport: <c>https://</c> = TLS, <c>http://</c> = h2c plaintext.
     ///     Required when <see cref="Enabled"/> is <c>true</c>.
     /// </summary>
+    /// <summary>
+    ///     Selects the historian backend provider (case-insensitive). Default <c>"Gateway"</c>; the Host's
+    ///     <c>HistorianBackendRegistry</c> owns the set of valid names. Gateway-specific keys
+    ///     (<see cref="Endpoint"/>, <see cref="ApiKey"/>, <see cref="UseTls"/>) are only required for <c>Gateway</c>.
+    /// </summary>
+    public string Backend { get; init; } = GatewayBackendName;
+
+    /// <summary>The default backend name.</summary>
+    public const string GatewayBackendName = "Gateway";
+
+    /// <summary>True when <see cref="Backend"/> selects the gateway backend (case-insensitive).</summary>
+    public bool IsGatewayBackend =>
+        string.Equals(Backend?.Trim(), GatewayBackendName, StringComparison.OrdinalIgnoreCase);
+
     public string Endpoint { get; init; } = "";
 
     /// <summary>
@@ -112,6 +126,9 @@ public sealed class ServerHistorianOptions
     {
         var warnings = new List<string>();
         if (!Enabled) return warnings;
+        // Gateway-specific checks apply only to the Gateway backend (other backends carry their own options).
+        if (IsGatewayBackend)
+        {
         if (string.IsNullOrWhiteSpace(Endpoint))
             warnings.Add("ServerHistorian:Endpoint is empty while the historian is enabled — the read client has no gateway address to dial.");
         // archreview 06/S-11 warning parity: a non-empty but unparseable / non-absolute-http(s) Endpoint
@@ -123,6 +140,7 @@ public sealed class ServerHistorianOptions
             warnings.Add($"ServerHistorian:Endpoint ('{Endpoint}') is not an absolute http(s) URI while the historian is enabled — the read client cannot dial it (e.g. https://host:5222).");
         if (string.IsNullOrWhiteSpace(ApiKey))
             warnings.Add("ServerHistorian:ApiKey is empty while the historian is enabled — the gateway gRPC surface will reject unauthenticated calls.");
+        }
         // MaxTieClusterOverfetch is intentionally checked AFTER the Enabled early-return above:
         // the over-fetch code path only runs when a real IHistorianDataSource is wired in,
         // so a zero/negative value is harmless (and noise-free) when the historian is disabled.