feat(site-call-audit): additive (UpdatedAtUtc, TrackedOperationId) keyset in the reconciliation pull contract

Site side: additive proto after_id field, ReadChangedSinceAsync gains an optional
afterId cursor and deterministic (UpdatedAtUtc, TrackedOperationId) ordering — a
batch fully inside one UpdatedAtUtc instant no longer re-reads the same page
forever. Absent/empty afterId preserves the exact legacy inclusive >= contract, so
an older central is unaffected. Regenerated the checked-in gRPC C#.

src/ZB.MOM.WW.ScadaBridge.Communication/Grpc/SiteStreamGrpcServer.cs

@@ -561,19 +561,24 @@ public class SiteStreamGrpcServer : SiteStreamService.SiteStreamServiceBase
             ? DateTime.SpecifyKind(request.SinceUtc.ToDateTime(), DateTimeKind.Utc)
             : DateTime.MinValue;
 
+        // Composite-keyset cursor (Task 15): proto3 defaults an unset string to
+        // "", which the store treats as "no cursor" (legacy inclusive >= behaviour)
+        // — so an older central that never sets after_id is unaffected.
+        var afterId = string.IsNullOrEmpty(request.AfterId) ? null : request.AfterId;
+
         IReadOnlyList<SiteCallOperational> operationals;
         try
         {
             operationals = await store.ReadChangedSinceAsync(
-                since, request.BatchSize, context.CancellationToken);
+                since, request.BatchSize, afterId, context.CancellationToken);
         }
         catch (Exception ex)
         {
             // Best-effort, like PullAuditEvents: a read fault must never abort
             // the reconciliation tick — central retries on its next cycle.
             _logger.LogError(ex,
-                "ReadChangedSinceAsync failed for since={Since} batch={Batch}; returning empty response.",
-                since, request.BatchSize);
+                "ReadChangedSinceAsync failed for since={Since} batch={Batch} afterId={AfterId}; returning empty response.",
+                since, request.BatchSize, afterId);
             return new PullSiteCallsResponse();
         }