Scroll::SortWindow

Maintains the top-N lines of the stream by the sort order, for --sort mode.

Rather than accumulate every line, it keeps at most capacity entries, sorted in display order (least-shown first, most-shown last). Each new line is keyed once; when the window is full and the new line is not greater than the current lowest kept entry, it is skipped with a single comparison. Otherwise it replaces the lowest entry and the window is re-sorted. So memory stays bounded at N and the common case (a line that does not make the cut) is O(1).

The window's contents are already in display order, so the render path draws snapshot directly — no further sorting needed.