feat(video): add timeline markers on V2 scrubber

[mrscobbler]

Summary

• Adds host-driven timeline markers on the V2 video scrubber
• Hosts push the marker list via a bp:timeline_markers_update window CustomEvent; clicks dispatch bp:timeline_marker_click for the host to route
• Viewer is display-only — no fetch, no parsing, no host reference
• Markers render as 2px white lines on the scrubber with a hover hit-target expansion (visual treatment is placeholder; final design TBD)

Test plan

• Open a video file with videoPlayerV2.enabled on
• Dispatch bp:timeline_markers_update with { detail: [{ id, timestampMs, type }, ...] } from a host or devtools; confirm white lines render at the right scrubber positions
• Click a marker; confirm bp:timeline_marker_click fires with the matching { id, timestampMs, type } payload
• On viewer setup, confirm bp:timeline_markers_ready is dispatched so hosts can replay their cached list
• Hover a marker; confirm the hit-target expansion makes the click more forgiving without changing the visible 2px line
• V1 viewers (videoPlayerV2.enabled off) — confirm no markers render, no events listened for

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