feat: timeline markers on the scrubber

[karngyan]

Adds optional timeline markers to Player and ScenesPlayer.

• New props: markers ({ id, time, color?, label? }[]) and onMarkerClick.
• Each marker is a small dot button on the scrubber. Hover shows its label in the scrub preview. Click calls onMarkerClick and does not seek.
• Markers outside the duration are skipped.
• Without markers, the DOM is the same as before (tested).
• Changeset: minor (0.13.0). README section and a demo example.

Chiron uses this to show reader comments on the chapter video.

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	kino	9a68aee	Commit Preview URL Branch Preview URL	Oct 07 2026, 01:47 PM