Initial commit: video effects processing system

Add S-expression based video effects pipeline with modular effect
definitions, constructs, and recipe files.

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

recipe-1.sexp

@@ -0,0 +1,16 @@
+; test recipe for language development
+(recipe "test"
+  :version "1.0"
+  :encoding (:codec "libx264" :crf 22 :preset "medium" :audio-codec "aac")
+
+  (effect invert
+    :path "../artdag-effects/invert/effect.py")
+
+  ; User-supplied input video
+  (def input-video
+    (-> (source :path "new.webm" :description "Input video")
+        (segment :start 0 :duration 3)))
+
+  (def branch-01 (-> input-video (fx invert :intensity 1.0)))
+
+  (sequence input-video branch-01))