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/pixelate.sexp

@@ -0,0 +1,11 @@
+;; Pixelate effect - creates blocky pixels
+;; @param block_size int [2, 64] default 8
+
+(define-effect pixelate
+  ((block_size 8))
+  (let* ((w (width frame))
+         (h (height frame))
+         (small-w (max 1 (floor (/ w block_size))))
+         (small-h (max 1 (floor (/ h block_size))))
+         (small (resize frame small-w small-h "area")))
+    (resize small w h "nearest")))