docs(mesh): Phase 6 — mark done, remove single-mesh caveats, document pair-local meshes

Redundancy.md: KNOWN-LIMITATION (per-Akka-cluster election) marked RESOLVED;
new 'Per-cluster meshes (Phase 6)' section + pair-local secrets note; manual-
failover mesh-scope caveat removed; auto-down 1-vs-1 note updated (every mesh is
now two nodes, drill deferred to Phase 7). IManualFailoverService + ClusterRedundancy
razor caveats updated to pair-local reality. Configuration.md documents the
cluster-{ClusterId} role, per-pair self-first seeds, SplitTopologyTransportValidator,
and the intentional Dps default. Program + design status tables + CLAUDE.md marked
Phase 6 DONE.

(Task 8 subagent completed the edits but died on an API error before committing;
reviewed and committed by the controller.)

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

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Components/Pages/Clusters/ClusterRedundancy.razor

@@ -68,10 +68,10 @@ else
             </div>
 
             <p class="text-muted small mt-2 mb-0">
-                Read live from cluster state on this node. <strong>Mesh-wide scope:</strong> the Primary is
-                elected once per Akka mesh, not per cluster row — in the current single-mesh topology this
-                acts on the whole mesh's Primary, which may be a node of another
-                <span class="mono">Cluster</span>. See <span class="mono">docs/Redundancy.md</span>.
+                Read live from cluster state on this node. The Primary shown is
+                <strong>this application <span class="mono">Cluster</span>'s own Primary</strong> — since
+                the per-cluster mesh split, each cluster's redundant pair runs its own independent 2-node
+                Akka mesh and elects its own Primary. See <span class="mono">docs/Redundancy.md</span>.
             </p>
 
             <AuthorizeView Policy="@ManualFailoverPageModel.RequiredPolicy">

src/Server/ZB.MOM.WW.OtOpcUa.ControlPlane/Redundancy/IManualFailoverService.cs

@@ -31,10 +31,9 @@ public sealed record ManualFailoverSnapshot(string? PrimaryAddress, IReadOnlyLis
 ///         moves with it and OPC UA clients re-select.
 ///     </para>
 ///     <para>
-///         <b>Mesh-scope caveat.</b> Until the per-cluster mesh work lands
-///         (<c>docs/plans/2026-07-21-per-cluster-mesh-design.md</c>) the Primary is elected once per
-///         Akka mesh, not per application <c>Cluster</c> row. On a fleet running several application
-///         clusters in one mesh this acts on the whole mesh's Primary. The UI says so.
+///         Post-Phase-6 (per-cluster mesh split): the Primary is elected per application
+///         <c>Cluster</c>, because each cluster's redundant pair now runs its own independent
+///         2-node Akka mesh. This service therefore acts on <b>this pair's own</b> Primary.
 ///     </para>
 /// </remarks>
 public interface IManualFailoverService