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/saturation.sexp

@@ -0,0 +1,7 @@
+;; Saturation effect - adjusts color saturation
+;; @param amount float [0, 3] default 1
+;; Uses vectorized shift-hsv primitive for fast processing
+
+(define-effect saturation
+  ((amount 1))
+  (shift-hsv frame 0 amount 1))