Auto: focas-f1c — Modal codes + overrides

Closes #259

Adds Modal/ + Override/ fixed-tree subfolders per FOCAS device, mirroring the
pattern established by Status/ (#257) and Production/ (#258): cached snapshots
refreshed on the probe tick, served from cache on read, no extra wire traffic
on top of user-driven tag reads.

Modal/ surfaces the four universally-present aux modal codes M/S/T/B from
cnc_modal(type=100..103) as Int16. **G-group decoding (groups 1..21) is deferred
to a follow-up** — the FWLIB ODBMDL union differs per series + group and the
issue body explicitly permits this scoping. Adds the cnc_modal P/Invoke +
ODBMDL struct + a generic int16 cnc_rdparam helper so the follow-up can add
G-groups without further wire-level scaffolding.

Override/ surfaces Feed/Rapid/Spindle/Jog from cnc_rdparam at MTB-specific
parameter numbers (FocasDeviceOptions.OverrideParameters; defaults to 30i:
6010/6011/6014/6015). Per-field nullable params let a deployment hide overrides
their MTB doesn't wire up; passing OverrideParameters=null suppresses the entire
Override/ subfolder for that device.

6 unit tests cover discovery shape, omitted Override folder when unconfigured,
partial Override field selection, cached-snapshot reads (Modal + Override),
BadCommunicationError before first refresh, and the FwlibFocasClient
disconnected short-circuit.

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

@@ -187,6 +187,61 @@ internal sealed class FwlibFocasClient : IFocasClient
         return true;
     }
 
+    private bool TryReadInt16Param(ushort number, out short value)
+    {
+        var buf = new FwlibNative.IODBPSD { Data = new byte[32] };
+        var ret = FwlibNative.RdParam(_handle, number, axis: 0, length: 4 + 2, ref buf);
+        if (ret != 0) { value = 0; return false; }
+        value = BinaryPrimitives.ReadInt16LittleEndian(buf.Data);
+        return true;
+    }
+
+    public Task<FocasModalInfo?> GetModalAsync(CancellationToken cancellationToken)
+    {
+        if (!_connected) return Task.FromResult<FocasModalInfo?>(null);
+        // type 100/101/102/103 = M/S/T/B (single auxiliary code, active modal block 0).
+        // Best-effort — if any single read fails we still surface the others as 0; the
+        // probe loop only updates the cache on a non-null return so a partial snapshot
+        // is preferable to throwing away every successful field.
+        return Task.FromResult<FocasModalInfo?>(new FocasModalInfo(
+            MCode: ReadModalAux(type: 100),
+            SCode: ReadModalAux(type: 101),
+            TCode: ReadModalAux(type: 102),
+            BCode: ReadModalAux(type: 103)));
+    }
+
+    private short ReadModalAux(short type)
+    {
+        var buf = new FwlibNative.ODBMDL { Data = new byte[8] };
+        var ret = FwlibNative.Modal(_handle, type, block: 0, ref buf);
+        if (ret != 0) return 0;
+        // For aux types (100..103) the union holds the code at offset 0 as a 2-byte
+        // value (<c>aux_data</c>). Reading as Int16 keeps the surface identical to the
+        // record contract; oversized values would have been truncated by FWLIB anyway.
+        return BinaryPrimitives.ReadInt16LittleEndian(buf.Data);
+    }
+
+    public Task<FocasOverrideInfo?> GetOverrideAsync(
+        FocasOverrideParameters parameters, CancellationToken cancellationToken)
+    {
+        if (!_connected) return Task.FromResult<FocasOverrideInfo?>(null);
+        // Each parameter is independently nullable — a null parameter number keeps the
+        // corresponding field at null + skips the wire call. A successful read on at
+        // least one parameter is enough to publish a snapshot; this matches the
+        // best-effort policy used by GetProductionAsync (issue #259).
+        var feed    = TryReadOverride(parameters.FeedParam);
+        var rapid   = TryReadOverride(parameters.RapidParam);
+        var spindle = TryReadOverride(parameters.SpindleParam);
+        var jog     = TryReadOverride(parameters.JogParam);
+        return Task.FromResult<FocasOverrideInfo?>(new FocasOverrideInfo(feed, rapid, spindle, jog));
+    }
+
+    private short? TryReadOverride(ushort? param)
+    {
+        if (param is null) return null;
+        return TryReadInt16Param(param.Value, out var v) ? v : null;
+    }
+
     // ---- PMC ----
 
     private (object? value, uint status) ReadPmc(FocasAddress address, FocasDataType type)