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/zoom.sexp

@@ -0,0 +1,6 @@
+;; Zoom effect - zooms in/out from center
+;; @param amount float [0.1, 5] default 1
+
+(define-effect zoom
+  ((amount 1))
+  (scale-img frame amount amount))