The ntc_temp2 lookup table was declared as 'char', which can be
unsigned on some platforms, causing negative temperature values
(-30°C to -1°C) to be incorrectly interpreted.
Changed to int8_t to ensure proper signed integer handling of
negative temperatures.
Fixes#1389
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
0084d92239