Add Echo tunnel, a video effect that flies through Feedback's nested echoes

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

src/effects/tunnel.hpp

@@ -0,0 +1,30 @@
+#pragma once
+
+#include "effects/video_effect.hpp"
+
+namespace effector {
+
+// The feedback effect in motion: the picture nested inside itself, each copy
+// smaller, turned and drifted further than the one around it, and the whole
+// stack flying inwards (or outwards) without end.
+//
+// A copy sits at a depth, and its size, turn and drift follow from that depth
+// alone. Every frame moves each copy on by part of one step, so after a whole
+// cycle each copy lies exactly where its neighbour started: a new one fades
+// in at the far end while the nearest fades out into the picture itself, and
+// the clip loops without a jump.
+class TunnelEffect : public VideoEffect {
+public:
+    std::string name() const override { return "tunnel"; }
+    std::string title() const override { return "Echo tunnel"; }
+    std::string description() const override {
+        return "Nests the picture inside itself, as Feedback does, and flies through the echoes "
+               "without end.";
+    }
+    const std::vector<ParamSpec>& params() const override;
+
+protected:
+    Image animate(Image input, const ParamValues& values, const Moment& at) const override;
+};
+
+}  // namespace effector