81b89faa3dd4b64e1ab775f72993a1da3484036f  Consolidate lazy load observers

Classified as not security-relevant.
Triage note: Merges LazyLoadImageObserver/VideoObserver/FrameObserver/ModelObserver into a single LazyLoadElementObserver with a shared lazyLoadIntersectionCallbackInvoked() callback; HTMLIFrameElement drops its own observer for m_isLazyLoading flags. Pure refactor/consolidation.
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_soft (+1.0), area:Source/WebCore/dom (+2.0), large_churn (-2.0)