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>

tests/core_tests.cpp

@@ -2108,6 +2108,8 @@ int main() {
         const std::vector<Moving> moving = {
             {"tunnel", {{"tunnel_drift", 6L}, {"tunnel_fade", 1.0}}, {}, {{"tunnel_fade", 0.0}}},
             {"flow", {{"flow_mode", std::string("swirl")}}, {}, {{"flow_strength", 0L}}},
+            {"sweep", {{"sweep_reverse", true}, {"sweep_max_run", 20L}}, {},
+             {{"sweep_line_chance", 0.0}}},
         };
         for (const auto& [name, lively, smooth, off] : moving) {
             const auto effect = effectByName(name);