Add Sort sweep, a video effect that sweeps Pixel sort's window over the brightness

The run sorting moves to effects/sort_runs.hpp so both share it; Pixel sort
renders exactly as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -21,6 +21,11 @@ commit that made it. The interface shows this file under Changelog in its menu.
   of noise as Warp does, with the field drifting so the picture ripples like
   heat haze or water. On the command line it is `-e flow` with
   `--flow-strength`, `--flow-travel`, `--flow-speed` and the rest.
+- Sort sweep, a video effect: runs of pixels sorted as Pixel sort sorts
+  them, only within a window of brightness that sweeps from dark to bright
+  and back, so the streaks spread over one part of the picture after
+  another. On the command line it is `-e sweep` with `--sweep-from`,
+  `--sweep-to`, `--sweep-width`, `--sweep-speed` and the rest.
 - A Quality choice under Video on Setup: Small file, Standard or High, for
   how the saved video is encoded. It is kept in the project, and on the
   command line it is `--video-quality small|standard|high`. Standard writes