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.

tests/StellaOps.Router.Common.Tests/FrameTypeTests.cs

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+using StellaOps.Router.Common;
+using Xunit;
+
+namespace StellaOps.Router.Common.Tests;
+
+public class FrameTypeTests
+{
+    [Fact]
+    public void FrameType_HasExpectedValues()
+    {
+        // Verify all expected frame types exist
+        Assert.True(Enum.IsDefined(typeof(FrameType), FrameType.Hello));
+        Assert.True(Enum.IsDefined(typeof(FrameType), FrameType.Heartbeat));
+        Assert.True(Enum.IsDefined(typeof(FrameType), FrameType.Request));
+        Assert.True(Enum.IsDefined(typeof(FrameType), FrameType.Response));
+        Assert.True(Enum.IsDefined(typeof(FrameType), FrameType.Cancel));
+    }
+
+    [Fact]
+    public void TransportType_HasExpectedValues()
+    {
+        // Verify all expected transport types exist
+        Assert.True(Enum.IsDefined(typeof(TransportType), TransportType.InMemory));
+        Assert.True(Enum.IsDefined(typeof(TransportType), TransportType.Tcp));
+        Assert.True(Enum.IsDefined(typeof(TransportType), TransportType.Tls));
+        Assert.True(Enum.IsDefined(typeof(TransportType), TransportType.Udp));
+        Assert.True(Enum.IsDefined(typeof(TransportType), TransportType.RabbitMq));
+    }
+
+    [Fact]
+    public void InstanceHealthStatus_HasExpectedValues()
+    {
+        // Verify all expected health statuses exist
+        Assert.True(Enum.IsDefined(typeof(InstanceHealthStatus), InstanceHealthStatus.Unknown));
+        Assert.True(Enum.IsDefined(typeof(InstanceHealthStatus), InstanceHealthStatus.Healthy));
+        Assert.True(Enum.IsDefined(typeof(InstanceHealthStatus), InstanceHealthStatus.Degraded));
+        Assert.True(Enum.IsDefined(typeof(InstanceHealthStatus), InstanceHealthStatus.Draining));
+        Assert.True(Enum.IsDefined(typeof(InstanceHealthStatus), InstanceHealthStatus.Unhealthy));
+    }
+}

tests/StellaOps.Router.Common.Tests/StellaOps.Router.Common.Tests.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <LangVersion>preview</LangVersion>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
+    <PackageReference Include="xunit" Version="2.9.2" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.0.1">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+    <PackageReference Include="coverlet.collector" Version="6.0.4">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+  </ItemGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\__Libraries\StellaOps.Router.Common\StellaOps.Router.Common.csproj" />
+  </ItemGroup>
+</Project>