Allow heat loop assignment in the editor

[maddavo]

Summary

Allow ModuleSystemHeat parts to be reassigned between heat loops while editing a vessel.

Problem

The currentLoopID selector was available in the editor, but ChangeLoop() returned immediately unless the scene was flight. Changing the value therefore updated the displayed/configured Loop ID without moving the module in the editor's live SystemHeatSimulator. Heat-loop calculations and the saved design could consequently disagree with the editor selection.

Change

• Accept loop changes in both the editor and flight scenes.
• Resolve the simulator lazily when the change event occurs.
• Return safely if the simulator is not available.
• Remove the module from its old loop and add it to the selected loop.

Verification

• Built SystemHeat.csproj in Release configuration.
• Build completed with zero errors and zero warnings.
• Compared against upstream master: one file changed.