feat(drivers): tag-set drift detection, gated on SupportsOnlineDiscovery

The second thing #516/§8.2 deliberately did not build. The original objection
stands — Modbus, S7, MQTT, AbLegacy and Sql echo authored config from
DiscoverAsync, so diffing discovered against authored is a tautology for them
— but it is now ENCODED rather than used as a reason not to build.

The discriminator already existed: ITagDiscovery.SupportsOnlineDiscovery is
documented as "enumerates the tag set from the live backend rather than
replaying pre-declared/authored tags", and it separates the fleet cleanly —
FOCAS, TwinCAT, MTConnect and AbCip true; the five config-echo drivers
explicitly false. The check runs only for a driver that is
SupportsOnlineDiscovery AND reports the new AuthoredDiscoveryRefs. That is
nullable rather than empty-by-default on purpose: an empty collection
legitimately means "nothing authored, everything discovered is new", so
conflating the two would report total drift for a driver that never opted in.

Where the value is: AbCip and FOCAS browse a live backend but implement no
IRediscoverable, so before this they had no change signal at all. A periodic
compare is the only way to notice a PLC re-download.

A finding that changed the design: FOCAS, TwinCAT and AbCip all re-emit their
authored tags into the same DiscoverAsync stream as device-derived ones, so
the browse picker can show an operator their existing tags. Discovered is
therefore always a superset of authored, and a VANISHED tag can never appear
missing — one whole direction is structurally undetectable for three of the
four. Shipping a Vanished list that is permanently empty for them would have
been the half-inert seam this whole exercise removes. So the driver declares
DiscoveryStreamIncludesAuthoredTags, the detector does not compute the
undetectable half, and the operator message says "missing-tag detection
unavailable for this driver" rather than letting the absence of a missing-tag
clause read as reassurance. MTConnect is the only driver where both directions
work — its stream is purely probe-model-derived.

Behaviour: a failed browse is not drift (an unreachable device would otherwise
report every authored tag as vanished); a truncated capture is skipped for the
same reason; an unchanged drift is reported once rather than re-stamping its
timestamp; drift that resolves clears the prompt. Interval defaults to 5
minutes — it browses a real device, and a tag set changing is an engineering
event, not a runtime one. It reuses the Stage-2 surfacing, raising the same
/hosts re-browse prompt, and never rebuilds the served address space.

Comparison logic is a pure, separately-tested type rather than actor-inline.
14 new tests. The gate was verified load-bearing by deleting the
SupportsOnlineDiscovery half: the tautology-driver test goes red, and it
asserts discovery was never invoked rather than merely "no prompt raised",
which would have passed for the wrong reason if the timer never fired.

Full suite green except the same three pre-existing fixture-gated integration
suites, identical counts.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect/MTConnectDriver.cs

@@ -907,6 +907,19 @@ public sealed class MTConnectDriver : IDriver, IReadable, ISubscribable, ITagDis
     /// </remarks>
     public bool SupportsOnlineDiscovery => true;
 
+    /// <inheritdoc />
+    /// <remarks>The DataItem ids the authored raw tags bind. <c>DiscoverAsync</c> emits
+    /// <c>FullName: dataItem.Id</c> from the Agent's probe model, so the two sets are the same
+    /// vocabulary.</remarks>
+    public IReadOnlyCollection<string>? AuthoredDiscoveryRefs =>
+        Volatile.Read(ref _binding).RawPathsByDataItemId.Keys;
+
+    /// <inheritdoc />
+    /// <remarks>False — <c>DiscoverAsync</c> is built purely from the Agent's probe model and never
+    /// re-emits authored tags, so an authored DataItem that the Agent stopped publishing IS visible as
+    /// missing. MTConnect is currently the only driver where both drift directions are detectable.</remarks>
+    public bool DiscoveryStreamIncludesAuthoredTags => false;
+
     /// <inheritdoc/>
     /// <remarks>
     ///     <c>Once</c>, not the default <c>UntilStable</c>: <see cref="DiscoverAsync"/> streams the