docs: add threat model, crypto boundaries, and audit scope documents

Security audit preparation:
- Threat model with STRIDE analysis and 5 threat actors
- Crypto boundaries documenting all 11 primitives and key lifecycle
- Audit scope document for external security firms

docs/src/SUMMARY.md

@@ -124,6 +124,14 @@
 
 ---
 
+# Security
+
+- [Threat Model](security/threat-model.md)
+- [Cryptographic Boundaries](security/crypto-boundaries.md)
+- [Audit Scope](security/audit-scope.md)
+
+---
+
 # Contributing
 
 - [Coding Standards](contributing/coding-standards.md)