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/FocasDriverOptions.cs

@@ -17,11 +17,15 @@ public sealed class FocasDriverOptions
 ///     One CNC the driver talks to. <paramref name="Series"/> enables per-series
 ///     address validation at <see cref="FocasDriver.InitializeAsync"/>; leave as
 ///     <see cref="FocasCncSeries.Unknown"/> to skip validation (legacy behaviour).
+///     <paramref name="OverrideParameters"/> declares the four MTB-specific override
+///     <c>cnc_rdparam</c> numbers surfaced under <c>Override/</c>; pass <c>null</c> to
+///     suppress the entire <c>Override/</c> subfolder for that device (issue #259).
 /// </summary>
 public sealed record FocasDeviceOptions(
     string HostAddress,
     string? DeviceName = null,
-    FocasCncSeries Series = FocasCncSeries.Unknown);
+    FocasCncSeries Series = FocasCncSeries.Unknown,
+    FocasOverrideParameters? OverrideParameters = null);
 
 /// <summary>
 ///     One FOCAS-backed OPC UA variable. <paramref name="Address"/> is the canonical FOCAS