cdfe0c37be223e40aeb95a9766b80fba4e0f5a5e  [line-clamp] Block boxes after the clamp point are still visible

Classified as not security-relevant.
Triage note: Implements CSS line-clamp container invisibility: Box::isVisible() consults usedStyle().visibility(), and LineClampUpdater gains isAfterClampPoint/setIsForcedHidden to mark boxes after the clamp point (and their out-of-flow descendants) hidden. CSS layout feature/correctness work, not a security 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), msg_medium (+2.0), msg_soft (+1.0), msg_noise (-2.5), area:Source/WebCore/layout (+2.0)