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>

effects/scanlines.sexp

@@ -0,0 +1,21 @@
+;; Scanlines effect - horizontal line overlay
+;; Usage: python3 plan.py effects/scanlines.sexp -p spacing=3 -p intensity=0.4 | python3 execute.py - -d . -o output.mp4
+;;
+;; Parameters:
+;;   spacing: pixels between lines (1 to 8, default: 3)
+;;   intensity: line darkness (0 to 1, default: 0.4)
+
+(recipe "scanlines"
+  :version "1.0"
+  :encoding (:codec "libx264" :crf 20 :preset "medium" :audio-codec "aac" :fps 30)
+
+  (effect scanlines :path "sexp_effects/effects/scanlines.sexp")
+
+  ;; Default parameters (overridden by -p spacing=N -p intensity=N)
+  (def spacing 3)
+  (def intensity 0.4)
+
+  (def video (source :path "monday.webm"))
+  (def clip (-> video (segment :start 0 :duration 10)))
+  (def result (-> clip (effect scanlines :spacing spacing :intensity intensity)))
+  result)