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the CONNECT packet — it cannot be added to a live session, changed, or withdrawn.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.Host/Drivers/DriverFactoryBootstrap.cs

@@ -55,7 +55,16 @@ public static class DriverFactoryBootstrap
             // The Galaxy driver resolves its Gateway.ApiKeySecretRef secret: arm through the
             // shared ISecretResolver (registered unconditionally by AddZbSecrets on every node).
             var secretResolver = sp.GetRequiredService<ISecretResolver>();
-            Register(registry, loggerFactory, secretResolver, scriptRoot);
+            // Gitea #511: the MQTT driver's Sparkplug primary-host claim is Primary-gated, and unlike
+            // every other gated operation it is a STANDING assertion on the driver's own connection —
+            // there is no inbound message for DriverHostActor to intercept — so the verdict has to
+            // reach the driver itself. Registered unconditionally on driver-role nodes by
+            // AddOtOpcUaRuntime. GetRequiredService, not GetService: a null here silently falls the
+            // driver back to "claim unconditionally", which on a redundant pair is the dual-claim this
+            // whole mechanism exists to prevent, and it would fail as a plant outage rather than as a
+            // startup error.
+            var primaryRoleSource = sp.GetRequiredService<IDriverPrimaryRoleSource>();
+            Register(registry, loggerFactory, secretResolver, primaryRoleSource, scriptRoot);
             return registry;
         });
         services.AddSingleton<IDriverFactory>(sp =>
@@ -142,6 +151,7 @@ public static class DriverFactoryBootstrap
         DriverFactoryRegistry registry,
         ILoggerFactory? loggerFactory,
         ISecretResolver secretResolver,
+        IDriverPrimaryRoleSource primaryRoleSource,
         ZB.MOM.WW.OtOpcUa.Core.Scripting.ScriptRootLogger? scriptRoot = null)
     {
         Driver.AbCip.AbCipDriverFactoryExtensions.Register(registry);
@@ -150,7 +160,7 @@ public static class DriverFactoryBootstrap
         Driver.FOCAS.FocasDriverFactoryExtensions.Register(registry);
         Driver.Galaxy.GalaxyDriverFactoryExtensions.Register(registry, secretResolver, loggerFactory);
         Driver.Modbus.ModbusDriverFactoryExtensions.Register(registry, loggerFactory);
-        Driver.Mqtt.MqttDriverFactoryExtensions.Register(registry, loggerFactory);
+        Driver.Mqtt.MqttDriverFactoryExtensions.Register(registry, loggerFactory, primaryRoleSource);
         // Tier A (the Register default): fully managed — HttpClient + System.Xml.Linq, no native SDK,
         // no COM. Tier C is the only tier that arms process-level recycle (MemoryRecycle hard-breach /
         // ScheduledRecycleScheduler), which would be wrong here: the driver is in-process and killing