`StartStopControl::handle_fc_on_causes_for_temp` bug

https://github.com/NatLabRockies/fastsim/pull/271#discussion_r3679328429

StartStopControl::handle_fc_on_causes_for_temp is intended to use a previous temperature (temp_prev) for hysteresis, but both tuple entries are populated from the same fc_temperature value. This makes the (temp_prev < temp_fc_forced_on && temperature < temp_fc_allowed_off) branch ineffective and effectively ignores the temp_fc_allowed_off threshold.

To implement the intended hysteresis, you likely need to track a previous fuel-converter temperature (e.g., add a temp_prev/temperature_prev field to FuelConverterThermalState and update it each step) and use that here; otherwise consider simplifying the logic and error message so it doesn’t imply a non-existent fc.temp_prev() API.