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/GatewayNodeConfig.cs

@@ -0,0 +1,28 @@
+namespace StellaOps.Gateway.WebService;
+
+/// <summary>
+/// Static configuration for a gateway node.
+/// </summary>
+public sealed class GatewayNodeConfig
+{
+    /// <summary>
+    /// Gets the region where this gateway is deployed (e.g., "eu1").
+    /// Routing decisions use this value; it is never derived from headers or URLs.
+    /// </summary>
+    public required string Region { get; init; }
+
+    /// <summary>
+    /// Gets the unique identifier for this gateway node (e.g., "gw-eu1-01").
+    /// </summary>
+    public required string NodeId { get; init; }
+
+    /// <summary>
+    /// Gets the environment name (e.g., "prod", "staging", "dev").
+    /// </summary>
+    public required string Environment { get; init; }
+
+    /// <summary>
+    /// Gets the neighbor regions for fallback routing, in order of preference.
+    /// </summary>
+    public IReadOnlyList<string> NeighborRegions { get; init; } = [];
+}

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 { }

src/Gateway/StellaOps.Gateway.WebService/StellaOps.Gateway.WebService.csproj

@@ -0,0 +1,13 @@
+<Project Sdk="Microsoft.NET.Sdk.Web">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <LangVersion>preview</LangVersion>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+  </PropertyGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\..\__Libraries\StellaOps.Router.Common\StellaOps.Router.Common.csproj" />
+    <ProjectReference Include="..\..\__Libraries\StellaOps.Router.Config\StellaOps.Router.Config.csproj" />
+  </ItemGroup>
+</Project>