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/contrast.sexp

@@ -0,0 +1,20 @@
+;; Contrast effect - adjust image contrast
+;; Usage: python3 plan.py effects/contrast.sexp -p amount=1.5 | python3 execute.py - -d . -o output.mp4
+;;
+;; Parameters:
+;;   amount: contrast multiplier (0.0 to 3.0, default: 1.5)
+;;           <1 = less contrast, >1 = more contrast
+
+(recipe "contrast"
+  :version "1.0"
+  :encoding (:codec "libx264" :crf 20 :preset "medium" :audio-codec "aac" :fps 30)
+
+  (effect contrast :path "sexp_effects/effects/contrast.sexp")
+
+  ;; Default parameter (overridden by -p amount=N)
+  (def amount 1.5)
+
+  (def video (source :path "monday.webm"))
+  (def clip (-> video (segment :start 0 :duration 10)))
+  (def result (-> clip (effect contrast :amount amount)))
+  result)