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.

tools/cosign/cosign.dev.key

@@ -0,0 +1,11 @@
+-----BEGIN ENCRYPTED SIGSTORE PRIVATE KEY-----
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+OCwicCI6MX0sInNhbHQiOiJ5dlhpaXliR2lTR0NPS2x0Q2M1dlFhTy91S3pBVzNs
+Skl3QTRaU2dEMTAwPSJ9LCJjaXBoZXIiOnsibmFtZSI6Im5hY2wvc2VjcmV0Ym94
+Iiwibm9uY2UiOiIyNHA0T2xJZnJxdnhPVnM3dlY2MXNwVGpkNk80cVBEVCJ9LCJj
+aXBoZXJ0ZXh0IjoiTHRWSGRqVi94MXJrYXhscGxJbVB5dkVtc2NBYTB5dW5oakZ5
+UUFiZ1RSNVdZL3lCS0tYMWdFb09hclZDWksrQU0yY0tIM2tJQWlJNWlMd1AvV3c5
+Q3k2SVY1ek4za014cExpcjJ1QVZNV3c3Y3BiYUhnNjV4TzNOYkEwLzJOSi84R0dN
+NWt1QXhJRWsraER3ZWJ4Tld4WkRtNEZ4NTJVcVJxa2NPT09vNk9xWXB4OWFMaVZw
+RjgzRElGZFpRK2R4K05RUnUxUmNrKzBtOHc9PSJ9
+-----END ENCRYPTED SIGSTORE PRIVATE KEY-----