fix(localdb): phase-2 live gate — 4 production defects found and fixed

Gate record: docs/plans/2026-07-20-localdb-phase2-live-gate.md.
Checks 1, 2, 5, 6 pass. Checks 3 and 4 are NOT satisfied — the defect they
were meant to confirm turned out to be the opposite of what the plan assumed.

Three defects crash-looped every driver node before check 1 could even run:

1. An empty ServerHistorian:ApiKey kills the host. ServerHistorianOptions-
   Validator exists to turn exactly that class of failure into a named
   OptionsValidationException, but its documented fail tier explicitly excluded
   ApiKey on the reasoning that a keyless client "degrades — the gateway rejects
   calls". It does not: the client validates its own options at construction, so
   the process dies during Akka startup and never makes a call.

2. UseTls disagreeing with the endpoint scheme kills the host too, in both
   directions (both messages confirmed in the shipped client assembly). Moving an
   endpoint from https to http without clearing UseTls is an ordinary migration
   slip.

3. Plaintext h2c was UNREACHABLE. HistorianGatewayClientAdapter forwarded the
   TLS-only options unconditionally, and AllowUntrustedServerCertificate defaults
   to false, so it always sent RequireCertificateValidation=true — which the
   client rejects outright when UseTls=false. Every http:// deployment crashed,
   though the scheme is documented as the supported way to select h2c, and the
   only workaround was to assert a certificate posture for a connection that has
   no certificate.

The fourth was the blocker, and it is Phase 2's own:

4. The drain gate deferred to a Primary that cannot deliver. Redundancy roles are
   elected CLUSTER-WIDE; the alarm queue is PAIR-LOCAL. On the rig the elected
   driver Primary is central-1 — it carries the driver Akka role, replicates
   nobody's LocalDb and does not even run the alarm historian — so every driver
   node logged "Historian drain suspended", including the two site-b nodes that
   have no peer at all. Nothing drained anywhere, where before Phase 2 it drained
   fine. The cost is not a duplicate; it is the buffer growing to the capacity
   wall and evicting the audit trail it exists to protect.

   Fixed in three layers: a separate ShouldDrainAlarmHistory policy (unknown role
   drains; the two gates now deliberately disagree, and a test pins that); peer-
   host matching in DriverHostActor so a node stands down only for a Primary
   holding its rows; and AddAlarmHistorian short-circuiting the gate when
   replication is unconfigured — testing BOTH Replication:PeerAddress and
   SyncListenPort, since only the dialing half sets the former while both halves
   share the queue.

   Every one of these follows from the asymmetry: a false allow costs a duplicate
   row, which at-least-once delivery already accepts and payload-hash ids
   collapse; a false deny loses data silently.

A third vacuous test, caught by the same delete-the-guard discipline: the
role-view tests stayed green with the guard removed, because AwaitAssert polls
until an assertion passes and the assertion was "reads open" — which is the
SEEDED value, satisfied at the first poll before the actor processed anything.
They now assert the sequence of published values through a recording view; the
control then goes red for exactly the cases that matter.

Migration evidence: 11 legacy rows across two deliberately overlapping files
converged to exactly 9 identical rows on both nodes, proving D-6's payload-hash
identity on real nodes rather than in a fixture.

Open design fork, recorded in the gate doc rather than decided here: a pair
cannot currently identify its own Primary, so both halves drain. Safe in every
topology — nothing loses data — but the gate's de-duplication benefit is
unrealised until roles are scoped per pair.

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Drivers/PrimaryGatePolicy.cs

@@ -25,4 +25,50 @@ public static class PrimaryGatePolicy
             RedundancyRole.Secondary or RedundancyRole.Detached => false,
             _ => driverMemberCount <= 1,   // unknown: allow only when no driver peer exists
         };
+
+    /// <summary>
+    ///     Decide whether this node should drain the replicated alarm store-and-forward queue.
+    /// </summary>
+    /// <remarks>
+    ///     <para>
+    ///         <b>Deliberately not <see cref="ShouldServiceAsPrimary"/>.</b> That gate protects a shared
+    ///         field device, where two nodes acting at once is dangerous and irreversible, so an unknown
+    ///         role with a peer present must deny. This gate protects an append-only historian, and the
+    ///         delivery contract is already <i>at-least-once across a failover, by design</i> — identical
+    ///         events even collapse to a single row, because ids hash the payload.
+    ///     </para>
+    ///     <para>
+    ///         The two error costs are therefore asymmetric: a false allow costs a duplicate history row;
+    ///         a false deny silently stops the alarm audit trail and eventually evicts it at the capacity
+    ///         wall. So this decision keys on the role <b>alone</b> and opens when the role is unknown —
+    ///         no membership tie-break, because cluster-wide driver count says nothing about whether
+    ///         <i>this</i> node has a redundant partner.
+    ///     </para>
+    ///     <para>
+    ///         Found by the LocalDb Phase 2 live gate: on a rig of four driver nodes in one cluster with
+    ///         no <c>Redundancy</c> section, borrowing the write gate's verdict suspended the drain on
+    ///         every node — including unpaired ones — so nothing drained anywhere.
+    ///     </para>
+    /// </remarks>
+    /// <param name="localRole">This node's last-known redundancy role, or <c>null</c> when unknown.</param>
+    /// <param name="queueSharingPeerIsPrimary">
+    ///     Whether the node holding <see cref="RedundancyRole.Primary"/> in the same snapshot is a node
+    ///     that <b>shares this node's queue</b> — i.e. its LocalDb replication partner.
+    ///     <para>
+    ///         Merely knowing that <i>a</i> Primary exists is not enough, because the redundancy role is
+    ///         a CLUSTER-WIDE election while the alarm queue is PAIR-LOCAL. A fleet runs many pairs in
+    ///         one cluster, so the elected driver Primary is usually in some other pair — and on the
+    ///         docker-dev rig it is a central node, which carries the driver Akka role, replicates
+    ///         nobody's LocalDb and does not even run the alarm historian. Deferring to it means this
+    ///         node's events are delivered by no one, ever.
+    ///     </para>
+    /// </param>
+    /// <returns><c>true</c> to drain; <c>false</c> only when a node holding these same rows is doing it.</returns>
+    public static bool ShouldDrainAlarmHistory(RedundancyRole? localRole, bool queueSharingPeerIsPrimary) =>
+        localRole switch
+        {
+            RedundancyRole.Primary => true,
+            RedundancyRole.Secondary or RedundancyRole.Detached => !queueSharingPeerIsPrimary,
+            _ => true,   // unknown role ⇒ drain; a duplicate row beats a silent gap
+        };
 }