fix(site-call-audit): composite keyset cursor eliminates the single-timestamp reconciliation pin; pinned state now a published event

Reconciliation cursor becomes composite (UpdatedAtUtc, TrackedOperationId);
IPullSiteCallsClient/GrpcPullSiteCallsClient forward the Task-15 after_id keyset
(additive param, null preserves the legacy inclusive >= contract). A burst
sharing one exact UpdatedAtUtc now drains via the id tiebreak instead of pinning
forever. A legacy site that ignores after_id is latched + published as
SiteCallReconciliationPinnedChanged on the EventStream (transition-only),
replacing the prior silent log line.

tests/ZB.MOM.WW.ScadaBridge.SiteCallAudit.Tests/SiteCallAuditPurgeTests.cs

@@ -42,7 +42,7 @@ public class SiteCallAuditPurgeTests : TestKit
     private sealed class NoOpPullClient : IPullSiteCallsClient
     {
         public Task<PullSiteCallsResponse> PullAsync(
-            string siteId, DateTime sinceUtc, int batchSize, CancellationToken ct) =>
+            string siteId, DateTime sinceUtc, string? afterId, int batchSize, CancellationToken ct) =>
             Task.FromResult(new PullSiteCallsResponse(Array.Empty<SiteCall>(), MoreAvailable: false));
     }