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/fixtures/rendering/index.ndjson

@@ -1 +1 @@
-{"template_id":"tmpl-incident-start","locale":"en-US","channel":"email","expected_hash":"TBD","body_sample_path":"tmpl-incident-start.email.en-US.json"}
+{"template_id":"tmpl-incident-start","locale":"en-US","channel":"email","expected_hash":"05eb80e384eaf6edf0c44a655ca9064ca4e88b8ad7cefa1483eda5c9aaface00","body_sample_path":"tmpl-incident-start.email.en-US.json"}

docs/notifications/fixtures/rendering/tmpl-incident-start.email.en-US.json

@@ -2,5 +2,5 @@
   "subject": "Incident started: ${incident_id}",
   "body": "Incident ${incident_id} started at ${started_at}. Severity: ${severity}.",
   "merge_fields": ["incident_id", "started_at", "severity"],
-  "preview_hash": "TBD"
+  "preview_hash": "05eb80e384eaf6edf0c44a655ca9064ca4e88b8ad7cefa1483eda5c9aaface00"
 }