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/Models/ReachabilityFactDocument.cs

@@ -23,6 +23,10 @@ public sealed class ReachabilityFactDocument
     [BsonElement("states")]
     public List<ReachabilityStateDocument> States { get; set; } = new();
 
+    [BsonElement("runtimeFacts")]
+    [BsonIgnoreIfNull]
+    public List<RuntimeFactDocument>? RuntimeFacts { get; set; }
+
     [BsonElement("metadata")]
     [BsonIgnoreIfNull]
     public Dictionary<string, string?>? Metadata { get; set; }
@@ -96,3 +100,24 @@ public sealed class ReachabilitySubject
         return string.Join('|', Component ?? string.Empty, Version ?? string.Empty).Trim('|');
     }
 }
+
+public sealed class RuntimeFactDocument
+{
+    [BsonElement("symbolId")]
+    public string SymbolId { get; set; } = string.Empty;
+
+    [BsonElement("codeId")]
+    [BsonIgnoreIfNull]
+    public string? CodeId { get; set; }
+
+    [BsonElement("loaderBase")]
+    [BsonIgnoreIfNull]
+    public string? LoaderBase { get; set; }
+
+    [BsonElement("hitCount")]
+    public int HitCount { get; set; }
+
+    [BsonElement("metadata")]
+    [BsonIgnoreIfNull]
+    public Dictionary<string, string?>? Metadata { get; set; }
+}