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/FwlibFocasClient.cs

@@ -139,6 +139,26 @@ internal sealed class FwlibFocasClient : IFocasClient
         }
     }
 
+    public Task<int> GetPathCountAsync(CancellationToken cancellationToken)
+    {
+        if (!_connected) return Task.FromResult(1);
+        var buf = new FwlibNative.ODBPATH();
+        var ret = FwlibNative.RdPathNum(_handle, ref buf);
+        // EW_FUNC / EW_NOOPT on single-path controllers — fall back to 1 rather than failing.
+        if (ret != 0 || buf.MaxPath < 1) return Task.FromResult(1);
+        return Task.FromResult((int)buf.MaxPath);
+    }
+
+    public Task SetPathAsync(int pathId, CancellationToken cancellationToken)
+    {
+        if (!_connected) return Task.CompletedTask;
+        var ret = FwlibNative.SetPath(_handle, (short)pathId);
+        if (ret != 0)
+            throw new InvalidOperationException(
+                $"FWLIB cnc_setpath failed with EW_{ret} switching to path {pathId}.");
+        return Task.CompletedTask;
+    }
+
     public Task<bool> ProbeAsync(CancellationToken cancellationToken)
     {
         if (!_connected) return Task.FromResult(false);