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/PathScopeSimulationEndpoint.cs

@@ -2,6 +2,7 @@ using System.Text;
 using System.Text.Json;
 using Microsoft.AspNetCore.Http.HttpResults;
 using Microsoft.AspNetCore.Mvc;
+using StellaOps.Policy.Engine.Options;
 using StellaOps.Policy.Engine.Streaming;
 using StellaOps.Policy.Engine.Overlay;
 
@@ -12,6 +13,7 @@ public static class PathScopeSimulationEndpoint
     public static IEndpointRouteBuilder MapPathScopeSimulation(this IEndpointRouteBuilder routes)
     {
         routes.MapPost("/simulation/path-scope", HandleAsync)
+            .RequireRateLimiting(PolicyEngineRateLimitOptions.PolicyName)
             .WithName("PolicyEngine.PathScopeSimulation");
 
         return routes;