bf83909d56bd  Share element intersection computation code between various clients, and prepare it for further reuse

Classified as not security-relevant.
Triage note: Moves computeClippedRectInRootContentsSpace into renderer helpers (computeClippedRectInMainFrameContentCoordinates/localBoundsForIntersection) and reexpresses the cross-origin scroll-margin cutoff with a lambda. The same-origin scroll-margin boundary logic is preserved; refactor, no bug fix.
This commit was flagged by the signal scorer and reviewed by AI (not a human). It is listed only so the triage is auditable; there is no root-cause analysis because this is not a vulnerability fix.

Structural signals: smartptr_member_added (+6.47), null_guard (+2.59), msg_noise (-2.5), area:Source/WebCore/rendering (+2.0), large_churn (-2.0)