feat(gps): add UM982 GPS driver replacing broken TinyGPS++
Implement a complete UM982 GNSS driver (um982_gps.h/.c) with:
- NMEA parser for GGA, RMC, THS, VTG with multi-talker support (GP/GN/GL/GA/GB)
- Correct coordinate parsing using decimal-point-based degree detection
(fixes PR #68 bug: 3-digit longitude degrees)
- Checksum verification on all incoming sentences
- Non-blocking line assembler with ring buffer
- Init sequence: UNLOG, HEADING FIXLENGTH, baseline config, NMEA enables,
VERSIONA handshake (no SAVECONFIG to avoid NVM wear)
- Validity/age checks with configurable timeouts
Integration into main.cpp:
- Replace TinyGPSPlus with UM982_GPS_t, UART5 baud 9600->115200
- Non-blocking um982_process() in main loop (single-byte UART reads)
- GPS heading override with magnetometer fallback
- Health check using um982_position_age()
Test infrastructure:
- 49 unit tests covering checksums, coordinate parsing, all sentence types,
talker IDs, feed/assembly, validity, init sequence, edge cases
- Mock HAL_UART_Receive with per-UART ring buffer for integration tests
- All 72 MCU tests passing (23 existing + 49 new)
Fixes all 12 bugs identified in PR #68 analysis (5 compile errors + 7 functional).
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