semi implemented and features implemented save checkpoint

docs/features/unchecked/exportcenter/oci-referrer-publishing.md

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+# OCI Referrer Publishing
+
+## Module
+ExportCenter
+
+## Status
+IMPLEMENTED
+
+## Description
+OCI referrer push client and discovery service for publishing attestations as OCI-attached artifacts.
+
+## Implementation Details
+- **Referrer push client**: `src/ExportCenter/StellaOps.ExportCenter/StellaOps.ExportCenter.WebService/Distribution/Oci/OciReferrerPushClient.cs` -- pushes attestations as OCI referrer artifacts
+- **Referrer discovery**: `src/ExportCenter/StellaOps.ExportCenter/StellaOps.ExportCenter.WebService/Distribution/Oci/OciReferrerDiscovery.cs` -- discovers attached referrer artifacts
+- **Referrer discovery service**: `src/ExportCenter/StellaOps.ExportCenter/StellaOps.ExportCenter.WebService/Distribution/Oci/OciReferrerDiscoveryService.cs` -- service wrapper for referrer discovery
+- **Referrer fallback**: `src/ExportCenter/StellaOps.ExportCenter/StellaOps.ExportCenter.WebService/Distribution/Oci/OciReferrerFallback.cs` -- fallback for tag-based referrer linking
+- **RVA publisher**: `src/ExportCenter/StellaOps.ExportCenter/StellaOps.ExportCenter.WebService/Distribution/Oci/RvaOciPublisher.cs` -- publishes RVA attestations as OCI referrers
+- **AI attestation publisher**: `src/ExportCenter/StellaOps.ExportCenter/StellaOps.ExportCenter.WebService/Distribution/Oci/AIAttestationOciPublisher.cs` -- AI attestation referrer publishing
+- **AI attestation discovery**: `src/ExportCenter/StellaOps.ExportCenter/StellaOps.ExportCenter.WebService/Distribution/Oci/AIAttestationOciDiscovery.cs` -- discovers AI attestation referrers
+- **Mirror referrer discovery**: `src/ExportCenter/StellaOps.ExportCenter/StellaOps.ExportCenter.Core/MirrorBundle/IReferrerDiscoveryService.cs`
+- **Tests**: `src/ExportCenter/StellaOps.ExportCenter/StellaOps.ExportCenter.Tests/Distribution/Oci/OciReferrerPushClientTests.cs`, `OciReferrerDiscoveryTests.cs`, `OciReferrerDiscoveryServiceTests.cs`, `RvaOciPublisherTests.cs`, `Adapters/MirrorAdapterReferrerDiscoveryTests.cs`
+- **Source**: Feature matrix scan
+
+## E2E Test Plan
+- [ ] Verify attestations are published as OCI referrer artifacts
+- [ ] Test referrer discovery finds attached artifacts for a digest
+- [ ] Verify fallback works for registries without referrer API
+- [ ] Test RVA and AI attestation referrer publishing
+- [ ] Verify mirror adapter discovers referrers across mirrors