Auto: focas-f2b — multi-path/multi-channel CNC

Adds optional `@N` path suffix to FocasAddress (PARAM:1815@2, R100@3.0,
MACRO:500@2, DIAG:280@2/1) with PathId defaulting to 1 for back-compat.
Per-device PathCount is discovered via cnc_rdpathnum at first connect and
cached on DeviceState; reads with PathId>PathCount return BadOutOfRange.
The driver issues cnc_setpath before each non-default-path read and
tracks LastSetPath so repeat reads on the same path skip the wire call.

Closes #264

tests/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FakeFocasClient.cs

@@ -61,6 +61,25 @@ internal class FakeFocasClient : IFocasClient
 
     public virtual Task<bool> ProbeAsync(CancellationToken ct) => Task.FromResult(ProbeResult);
 
+    /// <summary>
+    ///     Configurable path count surfaced via <see cref="GetPathCountAsync"/> — defaults to
+    ///     1 (single-path controller). Tests asserting multi-path behaviour set this to 2..N
+    ///     so the driver's PathId validation + cnc_setpath dispatch can be exercised
+    ///     without a live CNC (issue #264).
+    /// </summary>
+    public int PathCount { get; set; } = 1;
+
+    /// <summary>Ordered log of <c>cnc_setpath</c> calls observed on this fake session.</summary>
+    public List<int> SetPathLog { get; } = new();
+
+    public virtual Task<int> GetPathCountAsync(CancellationToken ct) => Task.FromResult(PathCount);
+
+    public virtual Task SetPathAsync(int pathId, CancellationToken ct)
+    {
+        SetPathLog.Add(pathId);
+        return Task.CompletedTask;
+    }
+
     public virtual void Dispose()
     {
         DisposeCount++;