Keep SystemHeat PAW stable when flux changes

[maddavo]

Fixes #151

Summary

Keep the SystemHeat part-action-window layout stable when system flux changes or briefly reaches zero in the editor.

Problem

ModuleSystemHeat.Update() was showing or hiding the System Temperature field every frame based on totalSystemFlux > 0. When the editor simulation temporarily changed the flux value, the field appeared and disappeared, causing the remaining PAW fields and controls to move continuously.

Change

Keep the System Temperature field visible while the SystemHeat module is enabled. Its displayed value continues to update normally, while module enable/disable state remains responsible for the field's visibility.

Verification

• Tested in KSP with the affected editor workflow.
• Built SystemHeat.csproj in Release configuration.
• Build completed with zero errors and zero warnings.
• Compared against upstream master: one file changed.