Add signal contracts for reachability, exploitability, trust, and unknown symbols

- Introduced `ReachabilityState`, `RuntimeHit`, `ExploitabilitySignal`, `ReachabilitySignal`, `SignalEnvelope`, `SignalType`, `TrustSignal`, and `UnknownSymbolSignal` records to define various signal types and their properties.
- Implemented JSON serialization attributes for proper data interchange.
- Created project files for the new signal contracts library and corresponding test projects.
- Added deterministic test fixtures for micro-interaction testing.
- Included cryptographic keys for secure operations with cosign.

src/Gateway/StellaOps.Gateway.WebService/Program.cs

@@ -0,0 +1,13 @@
+var builder = WebApplication.CreateBuilder(args);
+
+// Placeholder: Gateway services will be registered here in later sprints
+
+var app = builder.Build();
+
+// Placeholder: Middleware pipeline will be configured here in later sprints
+app.MapGet("/health", () => Results.Ok(new { status = "healthy" }));
+
+app.Run();
+
+// Make Program class accessible for integration tests
+public partial class Program { }