feat(mqtt): Sparkplug primary-host STATE publishing, Primary-gated (#511)

`ActAsPrimaryHost` was accepted by config and by the AdminUI form and did nothing but
log a warning. It now delivers the whole three-part contract or none of it: a retained
`ONLINE` on `spBv1.0/STATE/{hostId}`, a matching retained `OFFLINE` armed as the MQTT
Last Will, and an explicit `OFFLINE` on a clean stand-down.

**The claim is a dedicated connection, not a message.** A Will can only be registered in
the CONNECT packet — it cannot be added to a live session, changed, or withdrawn. So a
claim that is acquired and released at runtime cannot ride the long-lived ingest session.
`SparkplugPrimaryHost` opens its own connection (client id `{clientId}-spbhost`, forced
clean-session) when the node gains Primary and closes it when the node loses Primary; the
connection's lifetime *is* the claim, so every leg follows from its lifecycle. Ingest is
untouched by a role change — no dropped subscriptions, no lost birth cache, no data gap at
the Secondary->Primary transition.

**Primary-gated** (owner decision, recorded on the issue). Both pair nodes subscribe
independently, so an ungated claim means both claim the same host id — and the sharp
failure is not the two fighting over the retained STATE, it is that the SECONDARY's death
fires its Will and publishes OFFLINE while the Primary is healthy, stopping every edge node
in the group. Registering the Will only on whichever node is Primary at connect time fails
the other way: a node that later gains Primary would publish ONLINE with no Will behind it.

New seam: `IDriverPrimaryRoleSource` (`Core.Abstractions`) carrying the *data-plane* verdict
(`PrimaryGatePolicy.ShouldServiceAsPrimary`), not the alarm-drain verdict — an unknown role
with a driver peer present must deny. Registered unconditionally by `AddOtOpcUaRuntime`,
deliberately NOT alongside `IRedundancyRoleView`, which `AddAlarmHistorian` registers only
when that section is enabled: piggybacking would leave a pair with no alarm historian
falling back to "claim unconditionally". Polled on a 1s timer rather than evented — a
process singleton raising events at per-instance drivers outlives every driver it holds.

Accepted trade: a brief edge-visible OFFLINE window during failover. That is a genuine
state transition, not a glitch to hide.

Live-gated 5/5 against the Mosquitto fixture, including the Will leg — `AbandonForTests`
drops the socket with no DISCONNECT, because a clean disconnect makes the broker discard
the Will by protocol rule, so a test driven through `StopAsync` would observe the
stand-down OFFLINE and pass against an implementation that never armed a Will. Falsified:
removing the Will turns that leg and only that leg red.

Also corrects two stale claims found en route: the AdminUI form still said MQTT tags are
read-only (#508 shipped), and the driver doc still said nothing consumes `IRediscoverable`.

Claude-Session: https://claude.ai/code/session_015p7wGqy3YpZNCpDzTpGMKo

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Redundancy/IRedundancyRoleView.cs

@@ -1,3 +1,4 @@
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
 using ZB.MOM.WW.OtOpcUa.Runtime.Drivers;
 
 namespace ZB.MOM.WW.OtOpcUa.Runtime.Redundancy;
@@ -39,8 +40,22 @@ public interface IRedundancyRoleView
     void Publish(bool shouldDrainAlarmHistory);
 }
 
-/// <summary>Thread-safe <see cref="IRedundancyRoleView"/> backed by a volatile field.</summary>
-public sealed class RedundancyRoleView : IRedundancyRoleView
+/// <summary>
+///     Thread-safe <see cref="IRedundancyRoleView"/> + <see cref="IDriverPrimaryRoleSource"/>, each
+///     backed by its own volatile field.
+/// </summary>
+/// <remarks>
+///     <para>
+///         <b>Two verdicts, one object, on purpose.</b> Both are published from the same
+///         <c>DriverHostActor</c> snapshot handler and both are read off-actor, so a second class
+///         would duplicate the publish plumbing to hold a second bool. They are kept as distinct
+///         <i>properties</i> rather than collapsed into one because they are genuinely different
+///         decisions from different policies — see <see cref="IRedundancyRoleView"/>'s remarks on why
+///         the drain gate must open on an unknown role where the data-plane gate closes. Collapsing
+///         them is the bug that live gate already found once.
+///     </para>
+/// </remarks>
+public sealed class RedundancyRoleView : IRedundancyRoleView, IDriverPrimaryRoleSource
 {
     // Seeded with the decision for "role unknown" rather than a bare false. An unpublished view and a
     // node whose role nothing ever reports are genuinely the same situation, and they must behave the
@@ -49,9 +64,22 @@ public sealed class RedundancyRoleView : IRedundancyRoleView
     private volatile bool _shouldDrainAlarmHistory =
         PrimaryGatePolicy.ShouldDrainAlarmHistory(localRole: null, queueSharingPeerIsPrimary: false);
 
+    // Seeded from the same policy that will later publish here, for the unknown-role/no-peer case:
+    // a node nothing has reported a role for, on a cluster with no driver peer, is a single-node
+    // deployment and may claim. The moment a snapshot arrives this is replaced by the real verdict,
+    // and a multi-driver cluster's unknown role denies.
+    private volatile bool _shouldActAsPrimary =
+        PrimaryGatePolicy.ShouldServiceAsPrimary(localRole: null, driverMemberCount: 1);
+
     /// <inheritdoc/>
     public bool ShouldDrainAlarmHistory => _shouldDrainAlarmHistory;
 
+    /// <inheritdoc/>
+    public bool ShouldActAsPrimary => _shouldActAsPrimary;
+
     /// <inheritdoc/>
     public void Publish(bool shouldDrainAlarmHistory) => _shouldDrainAlarmHistory = shouldDrainAlarmHistory;
+
+    /// <inheritdoc/>
+    public void PublishPrimaryRole(bool shouldActAsPrimary) => _shouldActAsPrimary = shouldActAsPrimary;
 }