Add dog-invert-concat recipe

Dog video from cat + inverted user video concatenated.
Uses the new invert effect with frame-by-frame API.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

recipes/dog-invert-concat/recipe.sexp

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+; dog-invert-concat recipe
+; Creates dog video from cat, inverts a user-supplied video, then concatenates
+; Demonstrates: def bindings, variable inputs, effect with params, branching DAG
+
+(recipe "dog-invert-concat"
+  :version "1.0"
+  :description "Create dog video from cat, invert second video, concatenate"
+  :owner "@giles@artdag.rose-ash.com"
+
+  ; Registry
+  (asset cat
+    :hash "33268b6e167deaf018cc538de12dbe562612b33e89a749391cef855b320a269b"
+    :url "https://rose-ash.com/content/images/2026/01/cat.jpg")
+
+  (effect identity
+    :hash "640ea11ee881ebf4101af0a955439105ab11e763682b209e88ea08fc66e1cc03"
+    :url "https://github.com/gilesbradshaw/art-dag/tree/main/effects/identity")
+
+  (effect dog
+    :hash "d048fe313433eb4e38f0e24194ffae91b896ca3e6eed3e50b2cc37b7be495555"
+    :url "https://github.com/gilesbradshaw/art-dag/tree/main/effects/dog")
+
+  (effect invert
+    :hash "9144a60cdb73b9d3bf5ba2b4333e5b7e381ab02d2b09ee585375b4fa68d35327")
+
+  ; Create dog video from cat
+  (def dog-video
+    (-> (source cat)
+        (effect identity)
+        (effect dog)))
+
+  ; User-supplied second video, inverted
+  (def inverted-video
+    (-> (source :input "Second Video"
+                :description "Video to invert and concatenate after the dog video")
+        (fx invert :intensity 1.0)))
+
+  ; Concatenate: dog first, then inverted user video
+  (sequence dog-video inverted-video))