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/__Libraries/StellaOps.Microservice.SourceGen/Placeholder.cs

@@ -0,0 +1,13 @@
+namespace StellaOps.Microservice.SourceGen;
+
+/// <summary>
+/// Placeholder type for the source generator project.
+/// This will be replaced with actual source generator implementation in a later sprint.
+/// </summary>
+public static class Placeholder
+{
+    /// <summary>
+    /// Indicates the source generator is not yet implemented.
+    /// </summary>
+    public const string Status = "NotImplemented";
+}

src/__Libraries/StellaOps.Microservice.SourceGen/StellaOps.Microservice.SourceGen.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <LangVersion>preview</LangVersion>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+  </PropertyGroup>
+</Project>