Create Zephyr calculator application

## Goal
Build calculator app on Zephyr RTOS.

## Tasks
- [ ] Create `examples/calculator-zephyr/`
- [ ] Write C app with menu-driven interface
- [ ] Call WASM functions (compiled to ARM)
- [ ] Add CMakeLists.txt
- [ ] Test on QEMU

## Acceptance Criteria
- ✅ Zephyr app compiles
- ✅ Links with Synth-compiled WASM
- ✅ Runs on QEMU