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.

tests/reachability/StellaOps.Signals.Reachability.Tests/RuntimeFactsNdjsonReaderTests.cs

@@ -0,0 +1,49 @@
+using System.IO;
+using System.IO.Compression;
+using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
+using FluentAssertions;
+using StellaOps.Signals.Parsing;
+using Xunit;
+
+namespace StellaOps.Signals.Reachability.Tests;
+
+public sealed class RuntimeFactsNdjsonReaderTests
+{
+    [Fact]
+    public async Task ReadAsync_ParsesLines()
+    {
+        var ndjson = """
+{"symbolId":"sym::foo","hitCount":2}
+{"symbolId":"sym::bar","codeId":"elf:abcd","loaderBase":"0x1000","metadata":{"thread":"bg"}}
+
+""";
+
+        await using var stream = new MemoryStream(Encoding.UTF8.GetBytes(ndjson));
+        var events = await RuntimeFactsNdjsonReader.ReadAsync(stream, gzipEncoded: false, CancellationToken.None);
+
+        events.Should().HaveCount(2);
+        events[0].SymbolId.Should().Be("sym::foo");
+        events[1].LoaderBase.Should().Be("0x1000");
+    }
+
+    [Fact]
+    public async Task ReadAsync_HandlesGzip()
+    {
+        var ndjson = """
+{"symbolId":"sym::foo"}
+""";
+        await using var compressed = new MemoryStream();
+        await using (var gzip = new GZipStream(compressed, CompressionLevel.Optimal, leaveOpen: true))
+        await using (var writer = new StreamWriter(gzip, Encoding.UTF8, leaveOpen: true))
+        {
+            await writer.WriteAsync(ndjson);
+        }
+
+        compressed.Position = 0;
+
+        var events = await RuntimeFactsNdjsonReader.ReadAsync(compressed, gzipEncoded: true, CancellationToken.None);
+        events.Should().HaveCount(1);
+    }
+}