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/vignette.sexp

@@ -0,0 +1,21 @@
+;; Vignette effect - darkens corners
+;; @param strength float [0, 1] default 0.5
+;; @param radius float [0.5, 2] default 1
+
+(define-effect vignette
+  ((strength 0.5) (radius 1))
+  (let* ((w (width frame))
+         (h (height frame))
+         (cx (/ w 2))
+         (cy (/ h 2))
+         (max-dist (* (sqrt (+ (* cx cx) (* cy cy))) radius)))
+    (map-pixels frame
+      (lambda (x y c)
+        (let* ((dx (- x cx))
+               (dy (- y cy))
+               (dist (sqrt (+ (* dx dx) (* dy dy))))
+               (factor (- 1 (* (/ dist max-dist) strength)))
+               (factor (clamp factor 0 1)))
+          (rgb (* (red c) factor)
+               (* (green c) factor)
+               (* (blue c) factor)))))))