feat(wire): attribute updates carry source_execution_id/trace; PullTraceEvents RPC + site read-by-execution-ids

Flow Debugger (design 2026-09-05 §5.3/§5.4): AttributeValueUpdate fields 7/8 stamped by
StreamRelayActor and parsed by SiteStreamGrpcClient (older frames => null/false). New
PullTraceEvents unary RPC on SiteStreamService (same PSK-gated prefix as PullAuditEvents):
a point read of the site audit SQLite by ExecutionId OR ParentExecutionId
(ISiteAuditQueue.ReadByExecutionIdsAsync, json_extract on the codec's camelCase keys, new
IX_audit_event_occurred index, 64-id / 2000-row caps) that moves no cursor and flips no
ForwardState. Central IPullTraceEventsClient/GrpcPullTraceEventsClient mirror the audit pull
client (endpoint resolution, NodeB failover, fault => empty) and register beside it.

Claude-Session: https://claude.ai/code/session_01GSLkkHKz9JrAtmJjmQK3cP

tests/ZB.MOM.WW.ScadaBridge.AuditLog.Tests/Site/SiteAuditRetentionServiceTests.cs

@@ -127,6 +127,10 @@ public class SiteAuditRetentionServiceTests
     /// <summary>Stub queue that records purge cutoffs and can throw on the first tick.</summary>
     private sealed class RecordingSiteAuditQueue : ISiteAuditQueue
     {
+        public Task<IReadOnlyList<AuditEvent>> ReadByExecutionIdsAsync(
+            IReadOnlyCollection<Guid> executionIds, DateTime sinceUtc, int limit, CancellationToken ct = default)
+            => throw new NotSupportedException("not exercised by the retention service");
+
         private readonly ConcurrentQueue<DateTime> _purgeCalls = new();
         private int _calls;