7067ca63e913  [visionOS] Enter Exit Spatial Browsing switching videos leads to Greyed Out Playback UI

Classified as not security-relevant.
Triage note: HTMLMediaElement plumbing: adds seekableRangesChanged() notification wiring (on durationChanged, mediaPlayerSeekableTimeRangesChanged, and pushing current state to newly added clients). Client-notification plumbing with no memory-safety or logic security 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 (+2.59), uaf_lifetime (+4.75), area:Source/WebCore/html (+2.0), large_churn (-2.0)