Further reduce allocation overhead in ScrollingHitObjectContainer

[EVAST9919]

• Depends on framework package with Add readonly dictionary wrapper that removes enumerator alloc osu-framework#6163

Part 2 of #26788

Not sure how safe is making dictionary public though. Would've made it protected, however some mods (such as Depth) will benefit from using that instead of AliveObjects.

master	pr

[smoogipoo]

I've changed it to use a ReadOnlyDictionary<> since I don't feel too good about exposing as a Dictionary<>. It may also reduce some allocations due to linq queries in the other places I've adjusted.

[EVAST9919]

I've changed it to use a ReadOnlyDictionary

With that half of the allocation came back (1st item)

But I guess there's nothing we can do with that for now?

[smoogipoo]

[rant]
What... I looked into the source and it looked like it exposed the enumerator as the raw struct, but now I check again and it's actually boxing it. I can't believe this is still a thing even in .NET 8.
[/rant]

I've proposed ppy/osu-framework#6163 framework-side that could be used here.

see above