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.

etc/secrets/dsse-dev.signing.json

@@ -0,0 +1,6 @@
+{
+  "keyId": "notify-dev-hmac-001",
+  "secret": "ZGV2ZWxvcG1lbnQtc2lnbmluZy1rZXktZm9yLXRlc3Rpbmctb25seQ==",
+  "algorithm": "HMACSHA256",
+  "note": "Development-only HMAC key for DSSE signing. DO NOT use in production. Secret is base64 of 'development-signing-key-for-testing-only'."
+}