perf(historian): bounded-parallel per-tag drain (DrainConcurrency, default 8)

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

@@ -57,6 +57,21 @@ public sealed class ContinuousHistorizationOptions
     /// <summary>Steady drain cadence in seconds (also the post-success reschedule). Defaults to 2.</summary>
     public double DrainIntervalSeconds { get; init; } = 2;
 
+    /// <summary>
+    ///     Maximum historian tags a single drain pass writes concurrently. Defaults to 8; clamped to at
+    ///     least 1 by the recorder (1 restores the fully sequential drain).
+    ///     <para>
+    ///         One peeked batch is grouped per historian tag and each group is an independent gateway
+    ///         round-trip, so a sequential pass costs <c>tags × latency</c> — with a wide batch that is the
+    ///         dominant term in how fast the outbox can drain. The window bounds how many of those
+    ///         round-trips are outstanding at once, so a large tag count cannot flood the gateway. Nothing
+    ///         about the ack changes: the pass is still acked all-or-nothing after every group has
+    ///         reported, and the Gitea #538 per-tag quarantine still keys on "another group wrote this
+    ///         pass".
+    ///     </para>
+    /// </summary>
+    public int DrainConcurrency { get; init; } = 8;
+
     /// <summary>
     ///     Maximum un-acked outbox entries before the drop-oldest capacity policy kicks in; <c>0</c>
     ///     means unbounded.
@@ -102,6 +117,8 @@ public sealed class ContinuousHistorizationOptions
             warnings.Add("ContinuousHistorization:OutboxPath is empty while historization is enabled — the durable outbox has no directory to persist to; host startup will fail when the outbox is constructed. Set an absolute path on durable storage.");
         if (string.Equals(CommitMode, "Periodic", StringComparison.OrdinalIgnoreCase) && CommitIntervalMs <= 0)
             warnings.Add($"ContinuousHistorization:CommitIntervalMs is {CommitIntervalMs} — must be > 0 in Periodic commit mode; the periodic-commit loop cannot run.");
+        if (DrainConcurrency < 1)
+            warnings.Add($"ContinuousHistorization:DrainConcurrency is {DrainConcurrency} — must be >= 1; the recorder clamps it to 1 (a fully sequential drain).");
         return warnings;
     }
 }