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/ascii_art.sexp

@@ -0,0 +1,14 @@
+;; ASCII Art effect - converts image to ASCII characters
+;; @param char_size int [4, 32] default 8
+;; @param alphabet string default "standard"
+;; @param color_mode string default "color"
+;; @param contrast float [1, 3] default 1.5
+;; @param background list default (0 0 0)
+
+(define-effect ascii_art
+  ((char_size 8) (alphabet "standard") (color_mode "color") (contrast 1.5) (background (list 0 0 0)))
+  (let* ((sample (cell-sample frame char_size))
+         (colors (nth sample 0))
+         (luminances (nth sample 1))
+         (chars (luminance-to-chars luminances alphabet contrast)))
+    (render-char-grid frame chars colors char_size color_mode background)))