A/B Loop range should reset on new file

Expected behavior of the wanted feature

I've discussed this with @kasper93 on irc.

Currently mpv retains A/B loop range, even when a new file starts, I believe this behavior was overlooked, as it doesn't make sense to have the same loop range for different files.

As I've explained on irc, there might be a use case that made the behavior initially like this, I don't use this option often.

This originated from: https://github.com/Samillion/ModernZ/issues/724#issuecomment-4349574387

Alternative behavior of the wanted feature

Instead of plainly resetting A/B loop range on each file start, it can be tied to an option whether to keep it or reset it on file events.

Just in case there is a useful reason why it was kept and not reset between files.

Log File

N/A

Sample Files

N/A

The issue names no files or tests. Start by locating the A/B loop state and the file-start event handling, then verify how the range is retained across files. Done means the range resets when a new file starts, with behavior covered by an appropriate regression test; consider the optional-preservation alternative only if existing behavior has a documented use.