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.Signals.Contracts/Abstractions/ISignalConsumer.cs

@@ -0,0 +1,38 @@
+namespace StellaOps.Signals.Contracts;
+
+/// <summary>
+/// Interface for consuming signals from the signal bus.
+/// Implemented by services that process signals.
+/// </summary>
+public interface ISignalConsumer
+{
+    /// <summary>
+    /// Consumes signals from the signal bus as an async enumerable.
+    /// </summary>
+    /// <param name="filterType">Optional signal type to filter by.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>Async enumerable of signal envelopes.</returns>
+    IAsyncEnumerable<SignalEnvelope> ConsumeAsync(
+        SignalType? filterType = null,
+        CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Gets the latest signal for a given key.
+    /// </summary>
+    /// <param name="signalKey">The signal key to look up.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>The signal envelope if found, null otherwise.</returns>
+    ValueTask<SignalEnvelope?> GetLatestAsync(
+        string signalKey,
+        CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Gets all signals for a given PURL.
+    /// </summary>
+    /// <param name="purl">The package URL to look up.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>Collection of signal envelopes for the PURL.</returns>
+    ValueTask<IReadOnlyList<SignalEnvelope>> GetByPurlAsync(
+        string purl,
+        CancellationToken cancellationToken = default);
+}