fix(check): key content_overlap and text_occluded collapse by pair, and bill a clipped text box once (#2801)

Combines two fixes to the layout audit:

- `staticIssueKey` no longer includes `issue.text` for `content_overlap`
  and `text_occluded`; both builders now emit a unique selector for both
  ends, so the selector pair is the identity. An animating subject
  (count-up, typewriter, rotating word) no longer splits one held
  collision into transient groups.
- A clipping box that is its own nearest constraint reports
  `clipped_text` only, instead of also reporting `text_box_overflow`.
  A non-clipping self-constraint still reports.