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

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.)

Start by reading the CSS Text start/end-of-line processing rules and the CSS Ruby annotation/container definitions referenced in the issue. Compare whether processing should apply per annotation segment, at ruby edges, at line edges, or not at all, while accounting for CJK and Latin word separators; done means reaching a resolved specification behavior.