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/Policy/StellaOps.Policy.Engine/Endpoints/RiskSimulationEndpoints.cs

@@ -1,6 +1,7 @@
 using Microsoft.AspNetCore.Http.HttpResults;
 using Microsoft.AspNetCore.Mvc;
 using StellaOps.Auth.Abstractions;
+using StellaOps.Policy.Engine.Options;
 using StellaOps.Policy.Engine.Services;
 using StellaOps.Policy.Engine.Simulation;
 
@@ -12,6 +13,7 @@ internal static class RiskSimulationEndpoints
     {
         var group = endpoints.MapGroup("/api/risk/simulation")
             .RequireAuthorization()
+            .RequireRateLimiting(PolicyEngineRateLimitOptions.PolicyName)
             .WithTags("Risk Simulation");
 
         group.MapPost("/", RunSimulation)