Code Consistency and Quality Enhancements

[rawbytedev]

Overview: Improve API consistency, error handling, and implementation quality across all zerokv backends.

Tasks

• Rename NewMemDataBase() → NewMemDB() for clarity
• Standardize receiver naming to db in all implementations
• Normalize sync behavior documentation across backends
• Add input validation (nil/empty keys) in all implementations
• Add batch state validation (prevent post-Commit ops)
• Ensure consistent error wrapping