1f9701c40a1aba4deaccee1e30378bb3454f6cc8  [line-clamp] Block boxes after the clamp point still take up space

Classified as not security-relevant.
Triage note: Renames isHiddenByLineClamp->isForceHiddenByLineClamp and adds skipLayoutForForcedHidden so clamped-out boxes are not laid out and excluded from overflow. CSS line-clamp layout correctness feature; no memory-safety bug.
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 (+1.2), msg_medium (+2.0), msg_soft (+1.0), msg_noise (-2.5), area:Source/WebCore/rendering (+2.0)