a3d2b19de1762775a0db3494b2171356a53aa304  Image observer save/restore should be scope-bound

Classified as not security-relevant.
Triage note: Introduces ImageObserverDisableScope RAII guard and replaces manual setImageObserver(nullptr)/restore pairs in drawImage, BitmapImage::drawLuminanceMaskPattern and SVGImage. Functionally equivalent exception-safe refactor; no memory-safety bug fixed.
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 (+4.87), null_guard (+2.59), msg_soft (+1.0), msg_feature (-1.0), area:Source/WebCore/html (+2.0)