refactor(host): remove legacy site-psk-invalidation topic (#105)

Removes the #96 compatibility path that the unified central-cache-invalidation
topic (#100 item 3, 153609ef; compat 3a31dd2d) carried for one release:
- CentralCacheInvalidationBus no longer publishes a bare SitePskInvalidated
  on site-psk-invalidation; LegacySitePskTopic constant deleted.
- CentralCacheInvalidationSubscriber no longer subscribes to that topic or
  handles a bare SitePskInvalidated; it applies envelopes only.
central-cache-invalidation behaviour is unchanged (one envelope per
invalidation, origin filtering, per-handler fault isolation).

Gate met: 8e32f353 (carries 153609ef and 3a31dd2d) runs on every central
node - DEV vd03 (record 581e25ca) and QA vq03/vq04 (record 8ba5d5d7).

Mixed-version / rolling-upgrade constraint: a central node built before
153609ef listens only on site-psk-invalidation and no longer receives PSK
invalidations from a newer peer (only its TTL and the site-rejection refresh
bound the staleness). Mixed pairs across that boundary are no longer
supported; central pairs are stopped and started together anyway.
Documented in Component-Host.md and Component-Communication.md.

Tests: the legacy publish/subscribe tests are replaced by
PskRotation_OnOneNode_ReachesThePeersCaches_ThroughTheUnifiedTopic (node A
publishes, node B's PSK cache and secret-resolver cache are invalidated, and
nothing else is published) and a bare-message-ignored test with an envelope
positive control. The two-node cluster test now asserts the PSK invalidation
arrives exactly once and the publisher applies nothing.

Claude-Session: https://claude.ai/code/session_01GT67ACmpMR7b9LGRPLijMi

src/ZB.MOM.WW.ScadaBridge.Host/CentralCacheInvalidation.cs

@@ -26,11 +26,13 @@ namespace ZB.MOM.WW.ScadaBridge.Host;
 /// the subscriber skips its own.
 /// </para>
 /// <para>
-/// <b>Migration from <c>site-psk-invalidation</c>.</b> A <see cref="SitePskInvalidated"/> is also
-/// published, bare, on <see cref="LegacySitePskTopic"/>, and the subscriber still listens there,
-/// so a central pair that is briefly mixed-version during an upgrade keeps exchanging PSK
-/// invalidations. Applying a PSK invalidation twice is harmless. Remove both once every central
-/// node runs this build.
+/// <b>No legacy topic.</b> The #96 per-cache topic <c>site-psk-invalidation</c> was published to
+/// and subscribed for one release as a mixed-version bridge; it was removed in ScadaBridge#105
+/// once every central node ran a build with this topic. A central node built before
+/// ScadaBridge#100 item 3 listens only on the old topic and so no longer receives PSK
+/// invalidations from a newer peer (its TTL still bounds the staleness): the two central nodes
+/// of a pair must run builds that both carry this topic, which the stop-both/start-both
+/// upgrade of a central pair already guarantees.
 /// </para>
 /// </remarks>
 public sealed class CentralCacheInvalidationBus : ICentralCacheInvalidationPublisher
@@ -38,9 +40,6 @@ public sealed class CentralCacheInvalidationBus : ICentralCacheInvalidationPubli
     /// <summary>The single central cache-invalidation topic.</summary>
     public const string Topic = "central-cache-invalidation";
 
-    /// <summary>The #96 per-cache topic, kept for mixed-version central pairs (see remarks).</summary>
-    public const string LegacySitePskTopic = "site-psk-invalidation";
-
     private readonly ILogger<CentralCacheInvalidationBus> _logger;
     private volatile IActorRef? _mediator;
 
@@ -77,10 +76,6 @@ public sealed class CentralCacheInvalidationBus : ICentralCacheInvalidationPubli
         try
         {
             mediator.Tell(new Publish(Topic, new CentralCacheInvalidationEnvelope(OriginId, invalidation)));
-            if (invalidation is SitePskInvalidated psk)
-            {
-                mediator.Tell(new Publish(LegacySitePskTopic, psk));
-            }
         }
         catch (Exception ex)
         {
@@ -101,8 +96,7 @@ public sealed class CentralCacheInvalidationBus : ICentralCacheInvalidationPubli
 }
 
 /// <summary>
-/// Central actor that subscribes to <see cref="CentralCacheInvalidationBus.Topic"/> (and the
-/// legacy <see cref="CentralCacheInvalidationBus.LegacySitePskTopic"/>) and hands every
+/// Central actor that subscribes to <see cref="CentralCacheInvalidationBus.Topic"/> and hands every
 /// invalidation published by the peer central node to this node's
 /// <see cref="ICentralCacheInvalidationHandler"/>s.
 /// </summary>
@@ -127,8 +121,6 @@ public sealed class CentralCacheInvalidationSubscriber : ReceiveActor
                 Apply(envelope.Invalidation);
             }
         });
-        // Bare message on the #96 topic, from a peer that predates the shared topic.
-        Receive<SitePskInvalidated>(Apply);
         Receive<SubscribeAck>(_ => { });
     }
 
@@ -156,7 +148,6 @@ public sealed class CentralCacheInvalidationSubscriber : ReceiveActor
         {
             var mediator = DistributedPubSub.Get(Context.System).Mediator;
             mediator.Tell(new Subscribe(CentralCacheInvalidationBus.Topic, Self));
-            mediator.Tell(new Subscribe(CentralCacheInvalidationBus.LegacySitePskTopic, Self));
         }
         catch (Exception ex)
         {

src/ZB.MOM.WW.ScadaBridge.Host/SitePskRotationInvalidator.cs

@@ -108,9 +108,9 @@ public sealed class SitePskRotationInvalidator : ISecretCacheInvalidator, ICentr
 
 /// <summary>
 /// Peer message: the gRPC preshared key of <paramref name="SiteId"/> was rotated on a central node.
-/// Rides the central cache-invalidation topic; kept in this namespace because the #96
-/// <c>site-psk-invalidation</c> topic carries it bare by this type name (see
-/// <see cref="CentralCacheInvalidationBus"/>).
+/// Rides the central cache-invalidation topic inside a <see cref="CentralCacheInvalidationEnvelope"/>
+/// (see <see cref="CentralCacheInvalidationBus"/>); the #96 bare <c>site-psk-invalidation</c>
+/// topic was removed in ScadaBridge#105.
 /// </summary>
 /// <param name="SiteId">The site whose key changed.</param>
 public sealed record SitePskInvalidated(string SiteId) : ICentralCacheInvalidation;