Make obs validations/mutations against response configs more explicit

These are currently embedded into the dataframe creation logic, but should be split out and made explicit.

* Prefer doing mutations of response configs based on observations declarations instead of dataframes/dataframe creations.
* Prefer doing validations of obs configs based on response configs outside of dataframe creation, but rather closer to _observations.py

Start by locating the dataframe creation logic and read _observations.py to trace where observation declarations mutate response configs and where response configs validate observation configs. Separate these responsibilities from dataframe creation and place them near the observations logic; done means both validations and mutations follow the issue's stated configuration relationships.

Written by the indexing model from the issue text.