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/pixelate.sexp

@@ -0,0 +1,20 @@
+;; Pixelate effect - reduce resolution with blocky pixels
+;; Usage: python3 plan.py effects/pixelate.sexp -p block_size=16 | python3 execute.py - -d . -o output.mp4
+;;
+;; Parameters:
+;;   block_size: pixel block size (2 to 64, default: 16)
+;;               higher = more pixelated
+
+(recipe "pixelate"
+  :version "1.0"
+  :encoding (:codec "libx264" :crf 20 :preset "medium" :audio-codec "aac" :fps 30)
+
+  (effect pixelate :path "sexp_effects/effects/pixelate.sexp")
+
+  ;; Default parameter (overridden by -p block_size=N)
+  (def block_size 16)
+
+  (def video (source :path "monday.webm"))
+  (def clip (-> video (segment :start 0 :duration 10)))
+  (def result (-> clip (effect pixelate :block_size block_size)))
+  result)