Differentiate custom generators from default in ssg.py

It would be nice if we added a comment in the generated ssg.py to show which columns are being generated by default gens and which are being generated by custom ones. We could add a comment and perhaps some line breaks.

Start by locating the code that generates ssg.py and identify how default and custom generators are written into the output. Add comments and line breaks that clearly separate the two groups, then inspect the generated file to confirm the distinction is visible.

Written by the indexing model from the issue text.