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

tests/ZB.MOM.WW.ScadaBridge.Host.Tests/CentralCacheInvalidationBusTests.cs

@@ -44,7 +44,7 @@ public class CentralCacheInvalidationBusTests : TestKit
         var envelope = Assert.IsType<CentralCacheInvalidationEnvelope>(publish.Message);
         Assert.Equal(_bus.OriginId, envelope.OriginId);
         Assert.IsType<RoleMappingsInvalidated>(envelope.Invalidation);
-        mediator.ExpectNoMsg(TimeSpan.FromMilliseconds(100)); // only PSK also uses the legacy topic
+        mediator.ExpectNoMsg(TimeSpan.FromMilliseconds(100)); // one topic, one publish (#105)
     }
 
     /// <summary>Before the mediator exists publishing is a silent no-op.</summary>