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/crt.sexp

@@ -0,0 +1,28 @@
+;; CRT effect - old monitor simulation
+;; @param line_spacing int [1, 10] default 2
+;; @param line_opacity float [0, 1] default 0.3
+;; @param vignette float [0, 1] default 0.2
+
+(define-effect crt
+  ((line_spacing 2) (line_opacity 0.3) (vignette_amount 0.2))
+  (let* ((w (width frame))
+         (h (height frame))
+         (cx (/ w 2))
+         (cy (/ h 2))
+         (max-dist (sqrt (+ (* cx cx) (* cy cy)))))
+    (map-pixels frame
+      (lambda (x y c)
+        (let* (;; Scanline darkening
+               (scanline-factor (if (= 0 (mod y line_spacing))
+                                    (- 1 line_opacity)
+                                    1))
+               ;; Vignette
+               (dx (- x cx))
+               (dy (- y cy))
+               (dist (sqrt (+ (* dx dx) (* dy dy))))
+               (vignette-factor (- 1 (* (/ dist max-dist) vignette_amount)))
+               ;; Combined
+               (factor (* scanline-factor vignette-factor)))
+          (rgb (* (red c) factor)
+               (* (green c) factor)
+               (* (blue c) factor)))))))