Auto: focas-f2a — DIAG: address scheme

New FocasAreaKind.Diagnostic parsed from DIAG:nnn (whole-CNC) and
DIAG:nnn/axis (per-axis), validated against a per-series
FocasCapabilityMatrix.DiagnosticRange table (16i: 0-499; 0i-F family:
0-999; 30i/31i/32i: 0-1023; Power Motion i: 0-255; Unknown: permissive
per existing matrix convention).

IFocasClient gains ReadDiagnosticAsync(diagNumber, axisOrZero, type,
ct) with a default returning BadNotSupported so older transport
variants degrade gracefully. FwlibFocasClient implements it via a new
cnc_rddiag P/Invoke that reuses the IODBPSD struct (same shape as
cnc_rdparam). FocasDriver.ReadAsync dispatches Diagnostic addresses
through the new path; non-Diagnostic kinds keep the existing
ReadAsync route unchanged.

Tests: parser positives (DIAG:1031, DIAG:280/2, case-insensitive,
zero, axis-8) + negatives (malformed, axis>31), capability matrix
boundaries per series, driver-level dispatch verifying axis index
threads through, init-time rejection on out-of-range, and
BadNotSupported fallback when the wire client doesn't override the
default. 266/266 pass in Driver.FOCAS.Tests.

Closes #263

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -176,6 +176,25 @@ internal static class FwlibNative
         ref short outCount,
         ref IODBAXIS figureinfo);
 
+    // ---- Diagnostics ----
+
+    /// <summary>
+    ///     <c>cnc_rddiag</c> — read a CNC diagnostic value. <paramref name="number"/> is the
+    ///     diagnostic number (e.g. 1031 = current alarm cause); <paramref name="axis"/> is 0
+    ///     for whole-CNC diagnostics or the 1-based axis index for per-axis diagnostics.
+    ///     <paramref name="length"/> is sized like <see cref="RdParam"/> — 4-byte header +
+    ///     widest payload (8 bytes for Float64). The shape of the payload depends on the
+    ///     diagnostic; the managed side decodes via <see cref="FocasDataType"/> on the
+    ///     configured tag (issue #263).
+    /// </summary>
+    [DllImport(Library, EntryPoint = "cnc_rddiag", ExactSpelling = true)]
+    public static extern short RdDiag(
+        ushort handle,
+        ushort number,
+        short axis,
+        short length,
+        ref IODBPSD buffer);
+
     // ---- Currently-executing block ----
 
     /// <summary>