Add Security Audit Phase 9: comprehensive SSH security tests

- auth_security: password brute force, public key, user status, home dir
- crypto_security: AES-CTR, HMAC-SHA256, Curve25519, Ed25519
- file_access_security: path traversal, absolute path, symlink attack
- channel_security: window limits, request validation
- 18 new security tests, all pass (153 total)

markbase-core/src/security_audit/mod.rs

@@ -0,0 +1,9 @@
+mod auth_security;
+mod crypto_security;
+mod file_access_security;
+mod channel_security;
+
+pub use auth_security::*;
+pub use crypto_security::*;
+pub use file_access_security::*;
+pub use channel_security::*;