v2 scrolling performance and pending fixes

[luispabon]

Hey folks. I've been working on https://github.com/luispabon/steiner for a few months, a coding agent and as such heavy on scrolling. On upgrading to bubbletea v2, I came across a rather large drop in scrolling performance when side panels are present, which I addressed in two pull requests (one in bubbletea, one in ultraviolet), but I haven't had any luck getting them reviewed, and they're risking bitrot and me forgetting the details as well:

• perf: add DrawOver and HardScroll for incremental scroll rendering ultraviolet#130
• perf: scroll-optimised flush() in cursedRenderer #1725

Those two in tandem boost scrolling performance massively, although it's still not as fast as it was on bubbletea v1.x. I can do further digging after these are accepted.