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

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

@@ -162,6 +162,28 @@ public interface IFocasClient : IDisposable
     Task<(object? value, uint status)> ReadDiagnosticAsync(
         int diagNumber, int axisOrZero, FocasDataType type, CancellationToken cancellationToken)
         => Task.FromResult<(object?, uint)>((null, FocasStatusMapper.BadNotSupported));
+
+    /// <summary>
+    ///     Discover the number of CNC paths (channels) the controller exposes via
+    ///     <c>cnc_rdpathnum</c>. Multi-path CNCs (lathe + sub-spindle, dual-turret,
+    ///     etc.) report 2..N; single-path CNCs return 1. The driver caches the result
+    ///     once per device after connect + uses it to validate per-tag <c>PathId</c>
+    ///     values (issue #264). Default returns 1 so transports that haven't extended
+    ///     their wire surface keep behaving as single-path.
+    /// </summary>
+    Task<int> GetPathCountAsync(CancellationToken cancellationToken)
+        => Task.FromResult(1);
+
+    /// <summary>
+    ///     Switch the active CNC path (channel) for subsequent reads via
+    ///     <c>cnc_setpath</c>. Called by the driver before every read whose
+    ///     <c>FocasAddress.PathId</c> differs from the path most recently set on the
+    ///     session — single-path devices (PathId=1 only) skip the wire call entirely.
+    ///     Default is a no-op so transports that haven't extended their wire surface
+    ///     simply read whatever path the CNC has selected (issue #264).
+    /// </summary>
+    Task SetPathAsync(int pathId, CancellationToken cancellationToken)
+        => Task.CompletedTask;
 }
 
 /// <summary>