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/rotate.sexp

@@ -0,0 +1,8 @@
+;; Rotate effect - rotates image
+;; @param angle float [-360, 360] default 0
+;; @param speed float default 0 - rotation per second
+
+(define-effect rotate
+  ((angle 0) (speed 0))
+  (let ((total-angle (+ angle (* speed t))))
+    (rotate-img frame total-angle)))