Rewrite answer generator code to be more like the TS version

[gvanrossum]

This is a WIP:

• I haven't reviewed the mostly AI-generated code.
• So far search quality (as measured on the 69 Umesh questions) is significantly worse. It's trending downward: evals I ran in Sept-Oct had about 5 failures; mid January about 12; this branch has about 18 failures. I won't merge unless that gets better (at least as good as mid January, ideally as good or better as Sept-Oct).