feat: Implement Runtime Facts ingestion service and NDJSON reader

- Added RuntimeFactsNdjsonReader for reading NDJSON formatted runtime facts.
- Introduced IRuntimeFactsIngestionService interface and its implementation.
- Enhanced Program.cs to register new services and endpoints for runtime facts.
- Updated CallgraphIngestionService to include CAS URI in stored artifacts.
- Created RuntimeFactsValidationException for validation errors during ingestion.
- Added tests for RuntimeFactsIngestionService and RuntimeFactsNdjsonReader.
- Implemented SignalsSealedModeMonitor for compliance checks in sealed mode.
- Updated project dependencies for testing utilities.

src/Signals/StellaOps.Signals/Services/CallgraphIngestionService.cs

@@ -82,12 +82,13 @@ internal sealed class CallgraphIngestionService : ICallgraphIngestionService
                 ? null
                 : new Dictionary<string, string?>(request.Metadata, StringComparer.OrdinalIgnoreCase),
             Artifact = new CallgraphArtifactMetadata
-            {
-                Path = artifactMetadata.Path,
-                Hash = artifactMetadata.Hash,
-                ContentType = artifactMetadata.ContentType,
-                Length = artifactMetadata.Length
-            },
+        {
+            Path = artifactMetadata.Path,
+            Hash = artifactMetadata.Hash,
+            CasUri = artifactMetadata.CasUri,
+            ContentType = artifactMetadata.ContentType,
+            Length = artifactMetadata.Length
+        },
             IngestedAt = timeProvider.GetUtcNow()
         };
 
@@ -105,8 +106,8 @@ internal sealed class CallgraphIngestionService : ICallgraphIngestionService
             document.Nodes.Count,
             document.Edges.Count);
 
-        return new CallgraphIngestResponse(document.Id, document.Artifact.Path, document.Artifact.Hash);
-    }
+        return new CallgraphIngestResponse(document.Id, document.Artifact.Path, document.Artifact.Hash, document.Artifact.CasUri);
+    }
 
     private static void ValidateRequest(CallgraphIngestRequest request)
     {