7067ca63e9135059e8f2acd3f6e791a1c2e576c6  [visionOS] Enter Exit Spatial Browsing switching videos leads to Greyed Out Playback UI

Classified as not security-relevant.
Triage note: Wires a new seekableRangesChanged/seekableRangesFromMediaSourceChanged notification through HTMLMediaElement, MediaSourcePrivate and the platform media players so the fullscreen scrubber updates; functional plumbing with no memory-safety or 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)