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>

src/effects/focuspull.hpp

@@ -0,0 +1,27 @@
+#pragma once
+
+#include "effects/video_effect.hpp"
+
+namespace effector {
+
+// The blur effect in motion, as a lens being focused: either the whole
+// picture drifts out of focus and back, or a band of sharpness travels down
+// the picture and back up with the rest blurred, as a tilted lens shows it.
+//
+// The pull goes there and back once a cycle, so the clip loops without a
+// jump; nothing in it is random.
+class FocusPullEffect : public VideoEffect {
+public:
+    std::string name() const override { return "focuspull"; }
+    std::string title() const override { return "Focus pull"; }
+    std::string description() const override {
+        return "Blurs the picture as Blur does, pulling it out of focus and back, or moving a "
+               "band of sharpness up and down it.";
+    }
+    const std::vector<ParamSpec>& params() const override;
+
+protected:
+    Image animate(Image input, const ParamValues& values, const Moment& at) const override;
+};
+
+}  // namespace effector