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_zones.sexp

@@ -0,0 +1,28 @@
+;; ASCII Zones effect - different character sets for different brightness zones
+;; Dark areas use simple chars, mid uses standard, bright uses blocks
+;; @param char_size int [4, 32] default 8
+;; @param dark_threshold int [0, 128] default 80
+;; @param bright_threshold int [128, 255] default 180
+;; @param color_mode string default "color"
+
+(define-effect ascii_zones
+  ((char_size 8) (dark_threshold 80) (bright_threshold 180) (color_mode "color"))
+  (let* ((sample (cell-sample frame char_size))
+         (colors (nth sample 0))
+         (luminances (nth sample 1))
+         ;; Start with simple chars as base
+         (base-chars (luminance-to-chars luminances "simple" 1.2))
+         ;; Map each cell to appropriate alphabet based on brightness zone
+         (zoned-chars (map-char-grid base-chars luminances
+                        (lambda (r c ch lum)
+                          (cond
+                            ;; Bright zones: use block characters
+                            ((> lum bright_threshold)
+                             (alphabet-char "blocks" (floor (/ (- lum bright_threshold) 15))))
+                            ;; Dark zones: use simple sparse chars
+                            ((< lum dark_threshold)
+                             (alphabet-char " .-" (floor (/ lum 30))))
+                            ;; Mid zones: use standard ASCII
+                            (else
+                             (alphabet-char "standard" (floor (/ lum 4)))))))))
+    (render-char-grid frame zoned-chars colors char_size color_mode (list 0 0 0))))