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>

src/effects/sweep.hpp

@@ -0,0 +1,29 @@
+#pragma once
+
+#include "effects/video_effect.hpp"
+
+namespace effector {
+
+// The pixel sort in motion: only pixels inside a window of brightness are
+// sorted, and the window sweeps from the dark end to the bright end and back,
+// so the sorted streaks spread over one part of the picture after another,
+// melt away and return.
+//
+// Which lines are sorted, and how their runs are cut, is drawn from
+// at.steady() and stays put; the window alone moves, there and back once a
+// cycle, so the clip loops without a jump.
+class SweepEffect : public VideoEffect {
+public:
+    std::string name() const override { return "sweep"; }
+    std::string title() const override { return "Sort sweep"; }
+    std::string description() const override {
+        return "Sorts runs of pixels as Pixel sort does, only within a window of brightness that "
+               "sweeps from dark to bright and back.";
+    }
+    const std::vector<ParamSpec>& params() const override;
+
+protected:
+    Image animate(Image input, const ParamValues& values, const Moment& at) const override;
+};
+
+}  // namespace effector