76bf3bb6fcdfad5013fd389e8d2c72eaaf65e426  LayerOverlapMap stores redundant rects, making overlap tests slower than necessary

Classified as not security-relevant.
Triage note: RectList::append gains isAlreadyCovered/removeRecentRectsCoveredBy so recently-added overlap rects that contain or are contained by a new rect are collapsed; a compositing overlap-map performance optimization, no memory-safety or logic 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: clamp_saturate (+2.92), area:Source/WebCore/rendering (+2.0), surgical (+1.3)