Attempt to properly quantify the impact of mania Hard Rock / Easy mod application on overall difficulty

[bdach]

In osu!stable's osu!mania implementation, "Hard Rock" and "Easy" mods do not work the same way as they do on all other rulesets. Rather than applying a multiplier to the map's base Overall Difficulty value, it applies multipliers to the durations of hit windows themselves.

Applying a multiplier to the map's OD instead of the hit windows is not the same thing. The second thing is actually much harsher in magnitude, to the point where applying HR to any map is almost guaranteed to exceed "the effective OD" of 10, and applying EZ to any map is almost guaranteed to result in "negative effective OD".

Prior to the last release, osu!lazer was oblivious to this reality and just treated mania HR / EZ as it did every other ruleset. Last release, for the sake for gameplay parity across rulesets, the mods in question were adjusted to match stable, but in the process, it made it look on the surface like HR / EZ were not changing OD anymore.

In actuality, the hit windows were still being adjusted (now matching stable), but this was no longer visible in the OD displayed at song select.

This change attempts to convey that reality by displaying "effective OD", similar to what's already done in other rulesets when rate-changing mods are active. Note that the values this will display do not match stable and that is correct, because stable song select lies about the actual impact on OD by just assuming it can treat all rulesets in the same way.

---

Would close #34150 I guess.

And yes I would like all of the above to land on the changelog if possible if this is merged.

For further convincing that this makes any semblance of sense please see the following: https://www.desmos.com/calculator/yigt7jycdv

Citation for stable just lying about the OD. Also note that stable just lies about DT/HT affecting OD on mania as well although it doesn't quantify by how much because it only shows a down/up triangle.

[peppy]

Here's an attempted reword of the public message attached to this change which I think reads a bit better for end users:

In osu!stable's osu!mania implementation, "Hard Rock" and "Easy" mods do not work the same way as they do on all other rulesets. Rather than applying a multiplier to the map's base Overall Difficulty value, it applies multipliers to *the durations of hit windows themselves*.

Applying a multiplier to the map's OD instead of the hit windows is not the same thing. The second thing is actually *much harsher* in magnitude, to the point where applying HR to any map is almost guaranteed to exceed "the effective OD" of 10, and applying EZ to any map is almost guaranteed to result in "negative effective OD".

Prior to the last release, osu!lazer was oblivious to this reality and just treated mania HR / EZ as it did every other ruleset. Last release, for the sake for gameplay parity across rulesets, the mods in question were [adjusted to match stable](https://github.com/ppy/osu/pull/33982), but in the process, it made it look on the surface like HR / EZ were not changing OD anymore.

In actuality, the hit windows *were* still being adjusted (now matching stable), but this was no longer visible in the OD displayed at song select. 

This change attempts to convey that reality by displaying "effective OD", similar to what's already done in other rulesets when rate-changing mods are active. Note that the values this will display *do not match* stable *and that is correct*, because stable song select *lies* about the actual impact on OD by just assuming it can treat all rulesets in the same way.

Feel free to diff at your end and apply as you see fit.

[bdach]

Have applied.

[peppy]

seems fine to me