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>

sexp_effects/effects/blur.sexp

@@ -0,0 +1,6 @@
+;; Blur effect - gaussian blur
+;; @param radius int [1, 50] default 5
+
+(define-effect blur
+  ((radius 5))
+  (blur frame (max 1 radius)))