Test calculator on real hardware

## Goal
Deploy and test on physical board.

## Tasks
- [ ] Choose board: STM32F4 Discovery, nRF52, or RP2040
- [ ] Build for target
- [ ] Flash firmware
- [ ] Test interactively
- [ ] Document setup
- [ ] Record demo video

## Acceptance Criteria
- ✅ Runs on real hardware
- ✅ All operations work
- ✅ Photos/video of demo