Expose the Koheron CTL200 thermistor setpoint (rtset) as a settable HA
number in ohms, clamped to SEED_RT_SETPOINT_MIN/MAX_OHM. The former
read-only setpoint sensor is replaced by this control. Add a read-only
seed temperature in °C, computed in firmware from the actual thermistor
resistance via an NTC β-model (constants in config.h; update once the
DFB1550LP thermistor R25/β are confirmed).
Verified end-to-end over MQTT: set/clamp/restore of rtset and the °C
conversion (10 kΩ → 25.00 °C).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
