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/brightness.sexp

@@ -0,0 +1,7 @@
+;; Brightness effect - adjusts overall brightness
+;; @param amount float [-255, 255] default 0
+;; Uses vectorized adjust primitive for fast processing
+
+(define-effect brightness
+  ((amount 0))
+  (adjust frame amount 1))