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-rule-report.json

@@ -0,0 +1,28 @@
+{
+  "rule_id": "sample-rule",
+  "simulation_id": "sim-2025-12-04-001",
+  "executed_at": "2025-12-04T00:00:00Z",
+  "tenant_id": "test-tenant",
+  "fixtures_used": [
+    "docs/notifications/fixtures/rendering/tmpl-incident-start.email.en-US.json"
+  ],
+  "channels_tested": ["email", "slack"],
+  "results": {
+    "events_processed": 10,
+    "deliveries_simulated": 20,
+    "delivery_success": 20,
+    "delivery_failure": 0,
+    "quota_blocked": 0,
+    "redaction_applied": true
+  },
+  "determinism_check": {
+    "hash_algorithm": "blake3-256",
+    "output_digest": "0000000000000000000000000000000000000000000000000000000000000000",
+    "verified": true
+  },
+  "approval_required": true,
+  "approval_status": "pending",
+  "evidence_links": [
+    "docs/notifications/fixtures/rendering/index.ndjson"
+  ]
+}