Release v2.3.1

DroneCOT 2.3.1

Correctness release for CoT accuracy and identity.

- Fix: CoT ce/le reported ODID accuracy ENUM CODES as if they were metres.
  Code 0 means 'unknown' but rendered as ce=0, a perfect fix. Observed live:
  HorizAccuracy=9 (<30 m) drawn at 9 m; BaroAccuracy=0 as zero error.
- Fix: absent altitude rendered as sea level (<height value> defaulted to 0).
- Fix: MAC-less serial/MAVLink feeds shared one Unknown-BasicID_0 UID; now
  fall back to FEED-<sensor_id> so receivers stay distinct.