Problem

When a StreamingTextNode has more content than fits in the viewport, there is no visual indication that:

1. More content exists above or below the visible area
2. Where the current scroll position is relative to the total content
3. How to scroll (users may not discover PageUp/PageDown)

Expected Behavior

Add an optional vertical scrollbar to StreamingTextNode (and potentially other scrollable nodes):

1. Scrollbar track — thin column (1 char wide) on the right edge of the node
2. Scrollbar thumb — indicates current position within the total content
3. Visual style — configurable characters (e.g., │ for track, █ or ┃ for thumb) and colors
4. Proportional sizing — thumb size reflects viewport/content ratio
5. Auto-hide — only show scrollbar when content exceeds viewport height

API

var chatHistory = StreamingTextNode.Create()
    .WithScrollbar()  // Enable scrollbar with defaults
    // OR
    .WithScrollbar(new ScrollbarOptions
    {
        TrackChar = '│',
        ThumbChar = '█',
        TrackColor = Color.BrightBlack,
        ThumbColor = Color.White,
        AutoHide = true  // Only show when scrollable
    });

Existing Scroll Infrastructure

StreamingTextNode already has full scroll support via PersistedStreamBuffer:

• ScrollOffset property tracks position
• CanScrollUp / CanScrollDown properties
• MaxScroll computed from content vs viewport
• HandleInput() handles PageUp/PageDown/Ctrl+Home/Ctrl+End

The scrollbar would simply visualize this existing state during Render().

Reference

Terminal UIs like lazygit, btop, and k9s all use scrollbar indicators. The most common pattern is a single-character-wide track on the right edge.