Auto: s7-b2 — block-read coalescing for contiguous DBs

Closes #293

src/ZB.MOM.WW.OtOpcUa.Driver.S7/S7DriverOptions.cs

@@ -63,6 +63,24 @@ public sealed class S7DriverOptions
     ///     Running ↔ Stopped transitions.
     /// </summary>
     public S7ProbeOptions Probe { get; init; } = new();
+
+    /// <summary>
+    ///     Block-read coalescing gap-merge threshold (bytes). When two same-DB tags are
+    ///     within this many bytes of each other the planner folds them into a single
+    ///     <c>Plc.ReadBytesAsync</c> request and slices the response client-side. The
+    ///     default <see cref="S7BlockCoalescingPlanner.DefaultGapMergeBytes"/> = 16 bytes
+    ///     trades a minor over-fetch for one fewer PDU round-trip — over-fetching 16
+    ///     bytes is cheaper than the ~30-byte S7 request frame.
+    /// </summary>
+    /// <remarks>
+    ///     Raise the threshold for chatty PLCs where PDU round-trips dominate latency
+    ///     (S7-1200 with default 240-byte PDU); lower it when DBs are sparsely populated
+    ///     so the over-fetch cost outweighs the saved PDU. Setting to 0 disables gap
+    ///     merging entirely — only literally adjacent ranges (gap == 0) coalesce.
+    ///     STRING / WSTRING / CHAR / WCHAR / structured-timestamp / array tags always
+    ///     opt out of merging regardless of this knob.
+    /// </remarks>
+    public int BlockCoalescingGapBytes { get; init; } = S7BlockCoalescingPlanner.DefaultGapMergeBytes;
 }
 
 public sealed class S7ProbeOptions