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.

docs/notifications/simulations/sample-simulation-report.json

@@ -0,0 +1,13 @@
+{
+  "rule_id": "RULE-INCIDENT",
+  "tenant_id": "tenant-123",
+  "fixtures_version": "v1",
+  "result": "passed",
+  "evidence": [
+    {
+      "event_id": "evt-1",
+      "decision": "send",
+      "channel": "email"
+    }
+  ]
+}