Move required parameters out of the various opts arguments across all libraries

Required parameters should either be part of a separate 'params' object, or preferably (if there aren't too many of them) as separate arguments.

- [ ] 'add' method in grid -> pane-adder

Start with the issue's only listed item: the `add` method in `grid` and its `pane-adder` reference. Search the repository for these entry points and for `opts` arguments across the libraries, then determine the complete required-parameter scope. Done means required parameters are separated consistently and all affected callers and tests still pass.

Written by the indexing model from the issue text.