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/Options/SignalsAirGapOptions.cs

@@ -0,0 +1,49 @@
+using System;
+
+namespace StellaOps.Signals.Options;
+
+public sealed class SignalsAirGapOptions
+{
+    public SignalsSealedModeOptions SealedMode { get; } = new();
+
+    public void Validate()
+    {
+        SealedMode.Validate();
+    }
+}
+
+public sealed class SignalsSealedModeOptions
+{
+    public bool EnforcementEnabled { get; set; }
+
+    public string? EvidencePath { get; set; }
+
+    public TimeSpan MaxEvidenceAge { get; set; } = TimeSpan.FromHours(6);
+
+    public TimeSpan CacheLifetime { get; set; } = TimeSpan.FromMinutes(1);
+
+    public bool RequireEvidenceHealth { get; set; } = true;
+
+    public void Validate()
+    {
+        if (!EnforcementEnabled)
+        {
+            return;
+        }
+
+        if (string.IsNullOrWhiteSpace(EvidencePath))
+        {
+            throw new InvalidOperationException("Signals air-gap sealed-mode evidence path must be configured when enforcement is enabled.");
+        }
+
+        if (MaxEvidenceAge <= TimeSpan.Zero)
+        {
+            throw new InvalidOperationException("Signals air-gap sealed-mode max evidence age must be greater than zero.");
+        }
+
+        if (CacheLifetime <= TimeSpan.Zero)
+        {
+            throw new InvalidOperationException("Signals air-gap sealed-mode cache lifetime must be greater than zero.");
+        }
+    }
+}