feat: use secure buffers for HMAC and PBKDF2 secrets

[LimiNode]

Summary

• replace temporary HMAC buffers with secure_buffer and zeroize after use
• add secure_buffer support across PBKDF2 and time-token helpers
• expose vector/secure_buffer-first APIs for secrets and deprecate std::string overloads
• document PBKDF2/HKDF helpers and secure buffer utilities with Doxygen
• document secure_buffer overloads for PBKDF2 and TOTP validators
• document how to convert string API keys into secure_buffer in README files

Testing

• cmake -S . -B build
• cmake --build build
• ctest --test-dir build

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https://chatgpt.com/codex/tasks/task_e_68bb2c387c90832c875e8353fd80df9f