Fix osu!taiko slider velocity being written incorrectly to .osu file on export

[peppy]

• Depends on Change default slider velocity for new beatmaps to match osu!stable #25688 (just for sanity / sequence)
• Closes by default, Taiko base SV is not 1.40 #25674

Apart from fixing the writing to the file, I've also made what I'd consider a huge code quality improvement:

• The legacy constant has been moved into a local taiko context (and is no longer considered legacy.. which I think makes a lot more sense).
• The weird subclassing of a realm type which somehow worked is now gone.

Does this need test coverage? I'd argue not as now there is no special logic for taiko decode/encode when SliderMultiplier is concerned. Testing it would just look weird.

[bdach]

Does this need test coverage?

Judging by the amount of times we've had to touch this accursed thing and subsequently broke it in some accursed way? 100%. No matter how weird it looks.

[peppy]

Hmm, it's basically going to be opening the editor, exporting to an .osz file and testing the file contents' SliderMultiplier matches the bindable... unless you have something else in mind?

[bdach]

Sure. If that covers the incorrect encoding then that works.

[peppy]

Tests added!

[bdach]

!diffcalc
RULESET=taiko

[github-actions]

Target: #25689
Spreadsheet: https://docs.google.com/spreadsheets/d/1FbixkXNw6I4allhEMZs8JeEUIChsLFxhZziAvQDNqs0/edit