Support rich inline with `pre-wrap` whitespace preserve

Rich inline allows mixed formatted content, but it explicitly says that pre-wrap is not supported.

Is there a reason we can't support whitespace preservation? I'm working on a Canva style editor and I have a need for rich inline with whitespace.

Search the repository for the rich inline implementation and the documented pre-wrap limitation; inspect how whitespace is currently handled and measured. Reproduce the mixed-formatted-content case described in the issue, add coverage for preserved whitespace, and verify existing rich inline layout behavior remains intact.

Written by the indexing model from the issue text.