Add the second site→central RPC seam alongside the existing M2
IngestAuditEventsAsync. The Bundle D proto already lit up
IngestCachedTelemetry (CachedTelemetryBatch / IngestAck) so this commit
just plumbs the client-side abstraction:
* ISiteStreamAuditClient gains IngestCachedTelemetryAsync(batch, ct).
* NoOpSiteStreamAuditClient implements it returning an empty IngestAck
(same shape as M2 — production gRPC client lands in M6).
* SyncCallEmissionEndToEndTests' DirectActorSiteStreamAuditClient stub
throws NotSupportedException from the new method so a regression that
accidentally routes a cached packet through the sync stub fails loudly.
* New NoOpSiteStreamAuditClientTests cover the null-guard + empty-ack
contract for both batch shapes.
Bundle E task E1.
