fix(focas): PMC coalescing treats any rc refusal as per-tag fallback, skips transient busy memo, one connect per batch (#266)

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

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/IFocasClient.cs

@@ -84,7 +84,10 @@ public interface IFocasClient : IDisposable
     /// <param name="start">First byte address.</param>
     /// <param name="end">Last byte address, inclusive; a whole number of slots from <paramref name="start"/>.</param>
     /// <param name="cancellationToken">The cancellation token.</param>
-    /// <returns>The raw slot values (null when not Good) and the mapped OPC UA status.</returns>
+    /// <returns>
+    ///     The raw slot values (null when not Good) and the mapped OPC UA status. A returned non-Good status is
+    ///     always a CNC return-code refusal; a transport failure or timeout throws instead.
+    /// </returns>
     Task<(IReadOnlyList<long>? slots, uint status)> ReadPmcSlotsAsync(
         string letter, FocasDataType slotType, int start, int end, CancellationToken cancellationToken);