97577612f28b46ada2aad43e56138221cd26e135  AX: In isolated tree mode, a video's URL is never updated when resource selection picks a new source

Classified as not security-relevant.
Triage note: Adds AXObjectCache::onMediaElementCurrentSrcChanged and fires AXNotification::URLChanged from setCurrentSrc. Pure accessibility feature wiring; no bug addressed.
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 (+3.0), null_guard (+1.2), area:Source/WebCore/html (+2.0), surgical (+1.3)