Implement External Security Encryption Module for ATmega32U4

Select and integrate a suitable external security encryption module for securely storing device keys on the ATmega32U4. The module must support industry-standard encryption protocols and be fully compatible with our hardware. Deliver a tested and documented solution.

## Tasks
- [ ] Research and select a compatible encryption module
- [ ] Implement secure key storage on ATmega32U4
- [ ] Integrate module with existing firmware
- [ ] Test read/write operations and encryption functionality
- [ ] Document implementation and usage

## Acceptance Criteria
- Module selection is documented with justification
- Device keys are stored securely and can be accessed safely
- Firmware integration does not break existing features
- Implementation and test results are fully documented

Start by reviewing the existing ATmega32U4 firmware and hardware integration points, then research security modules compatible with the Arduino Pro Micro. Compare candidates against secure key storage, encryption support, and hardware compatibility; done means a documented selection, integrated firmware, read/write and encryption tests, and usage documentation.

Written by the indexing model from the issue text.