[Feature Request] Cheevos - Subset visibility in overlay notifications

Overview

There doesn't seem to be much in the way of achievement subset visibility in overlay notifications.

Since 1.22.1 supports simultaneous multiset achievement lists, it would be beneficial for users to have some visibility into how many sets they are currently playing / currently have active when they log in, as well as which set a given achievement unlock is for.

If the user has only one set active (either because the game still requires patched ROMs or has deactivated all but one subset), we could hide the extra set phrasing and leave current notifications unchanged.

Generalized suggestions (illustrative-only; exact formatting/wording up to experimentation):

You have _ of _ achievements unlocked

becomes

You have _ of _ achievements unlocked from _ active sets

OR

You have _ of _ achievements unlocked
(_ active sets)

[Rare ]Achievement Unlocked - xx.x%
xxxxxxxxx (xx)

becomes

[Rare ]Achievement Unlocked - xx.x%
Subset: xxxxxxxxx
xxxxxxxxx (xx)

Open question: if implemented, would we omit the subset line if the achievement is from the base set and only show it if it's from a subset; or always show the set name if multiple subsets are active?

Notes

Very open to opinions / suggestions. I am cognizant of the noise this change may introduce to the notifications, so open to putting this behind a settings flag and keeping existing behavior unchanged (though I think the number of sets active upon game launch is fairly minimal and very helpful for someone who is launching a game for the first time and may not realize they have multiple sets on that they don't want to have on)

I may fiddle around with a branch and post some mockups of set-aware notifications if/when I get some time.

Somewhat related subset visibility issue: https://github.com/libretro/RetroArch/issues/18618

No files, tests, or entry points are identified. Start by reviewing the existing overlay notification behavior and the related subset visibility issue (#18618); done requires an agreed design for active-set counts, achievement set labels, and the single-set case.