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/FwlibNative.cs

@@ -99,6 +99,19 @@ internal static class FwlibNative
     [DllImport(Library, EntryPoint = "cnc_rdtimer", ExactSpelling = true)]
     public static extern short RdTimer(ushort handle, short type, ref IODBTMR buffer);
 
+    // ---- Modal codes ----
+
+    /// <summary>
+    ///     <c>cnc_modal</c> — read modal information for one G-group or auxiliary code.
+    ///     <paramref name="type"/>: 1..21 = G-group N (single group), 100 = M, 101 = S,
+    ///     102 = T, 103 = B (per Fanuc FOCAS reference). <paramref name="block"/>: 0 =
+    ///     active modal commands. We only consume types 100..103 today (M/S/T/B); the
+    ///     G-group decode is deferred to a follow-up because the <c>ODBMDL</c> union
+    ///     varies by group + series (issue #259).
+    /// </summary>
+    [DllImport(Library, EntryPoint = "cnc_modal", ExactSpelling = true)]
+    public static extern short Modal(ushort handle, short type, short block, ref ODBMDL buffer);
+
     // ---- Structs ----
 
     /// <summary>
@@ -151,6 +164,22 @@ internal static class FwlibNative
         public int Msec;
     }
 
+    /// <summary>
+    ///     ODBMDL — single-group modal read buffer. 4-byte header + a 4-byte union which we
+    ///     marshal as a fixed byte array. For type=100..103 (M/S/T/B) the union holds an
+    ///     <c>int aux_data</c> at offset 0; we read the first <c>short</c> for symmetry with
+    ///     the FWLIB <c>g_modal.aux_data</c> width on G-group reads. The G-group decode
+    ///     (type=1..21) is deferred — see <see cref="Modal"/> for context (issue #259).
+    /// </summary>
+    [StructLayout(LayoutKind.Sequential, Pack = 1)]
+    public struct ODBMDL
+    {
+        public short Datano;
+        public short Type;
+        [MarshalAs(UnmanagedType.ByValArray, SizeConst = 8)]
+        public byte[] Data;
+    }
+
     /// <summary>ODBST — CNC status info. Machine state, alarm flags, automatic / edit mode.</summary>
     [StructLayout(LayoutKind.Sequential, Pack = 1)]
     public struct ODBST