[css-ruby] start/end-of-line white-space processing for annotations

[fantasai]

For bases, how css-text start/end-of-line processing white-space applies is clear, since the base is in the main inline formatting context. However, for annotations, it is less clear.

Is css-text's start/end-of-line processing applied per segment/annotation container, or only to the start-most/end-most annotation containers in the ruby, or only to annotations at the start/end of the line, or not at all?

(For CJK, it would be fine to strip all spaces, but for Latin-based annotations, word separators are sometimes necessary, so this needs to be solved in consideration of that context.)