Add Character Streaming text animation

[theontho]

Add an explicit Character Streaming rendering mode that smooths chunky server updates by buffering attributed composed graphemes and releasing them one at a time.

The mode should preserve Markdown styling, links, layout, accessibility, Reduce Motion behavior, completion/replacement handling, and existing Fade behavior across UIKit and AppKit. The sample should expose Character Streaming and Standard Fade as selectable modes.

[junyan72]

what's the context on this feature request?

[28pins]

Does the approach of #152 work? It isn't exactly the same, but it has the same idea.