In this Phase we extend Part 2 by adding GPS power control and porting the I²C slave to TinyWireS (USI-TWI) on an ATtiny85. The ATtiny85:
• Parses NMEA GGA from a GPS module on D3/D4
• Presents itself as an I²C slave at address 0x42 on A4/A5
• On each master request, sends one register (0–9) of GPS data in little-endian
• Responds to register 9 write (0/1) to switch GPS power via D6 → MOSFET/NPN