Add Focus pull, a video effect that pulls Blur in and out of focus

Blur's box blur moves to effects/box_blur.hpp so both share it; Blur renders
exactly as before.

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

tests/core_tests.cpp

@@ -2210,6 +2210,9 @@ int main() {
               {"gate_scratches", 0L}}},
             {"datamosh", {{"mosh_rate", 0.4}, {"mosh_block", 8L}, {"mosh_bleed", true}}, {},
              {{"mosh_rate", 0.0}}},
+            {"focuspull", {{"focus_radius", 5L}}, {}, {{"focus_radius", 0L}}},
+            {"focuspull", {{"focus_mode", std::string("band")}, {"focus_speed", 1.0}}, {},
+             {{"focus_mode", std::string("band")}, {"focus_radius", 0L}}},
         };
         for (const auto& [name, lively, smooth, off] : moving) {
             const auto effect = effectByName(name);