Optimized Credential Storage

What: Refactored credential storage from factory functions to singleton objects with memoized read operations.

Details:

• Keychain storage (Hy1) and plaintext storage (pZ1) now singleton objects
• Read operations memoized to avoid redundant file/keychain access
• Storage path computation cached via jV0() helper
• Explicit cache invalidation after update() or delete() ensures correctness
• Improved performance through reduced object allocation and I/O operations
• No user-visible changes to behavior
• Evidence: Keychain singleton at line 378779, plaintext singleton at line 378823, path memoization at line 378818