Complete Beatmapping/Lead-in

[TicClick]

(to the best of my knowledge; part of #7254)

unconfirmed:

• automatic 2s value (only comes from my measurements, unable to find it elsewhere) -- found https://osu.ppy.sh/community/forums/topics/155870?n=2 but the image is gone
• lead-in vs storyboards: I found https://github.com/ppy/osu/blob/7d5a74b281d4338193d76eb4b6a870432d1f4cbe/osu.Game/Screens/Play/MasterGameplayClockContainer.cs#L86-L87 and similar considerations for the stable build passed verbally, but I don't have an example of a storyboard with custom epilepsy warning at a negative timestamp to see how it works with the lead-in

Self-check

• The changes are tested against the contribution checklist