Implement radar power switching (MOSFET GPIO control)

[xishell]

Description

The Waveshare HMMD radar module draws ~50 mA continuously and has no built-in sleep or enable pin. Without power switching, battery life drops from ~13 days to ~2.3 days. Implement GPIO-controlled power switching using an N-channel MOSFET (e.g. 2N7002) on GPIO 0 to cut power to the radar during deep sleep.

Acceptance Criteria

• GPIO 0 HIGH powers on the radar, GPIO 0 LOW cuts power
• Radar draws near-zero current when MOSFET is off
• Radar stabilizes and produces valid readings within 2 seconds of power-on
• Power switching does not interfere with ESP32-C3 boot (GPIO 0 is a strapping pin)

Additional Context

MOSFET (e.g. 2N7002 SOT-23) is not yet ordered and needs to be sourced. See docs/node-hardware.md for circuit diagram and power budget impact.