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/threshold.sexp

@@ -0,0 +1,13 @@
+;; Threshold effect - converts to black and white
+;; @param level int [0, 255] default 128
+;; @param invert bool default false
+
+(define-effect threshold
+  ((level 128) (invert false))
+  (map-pixels frame
+    (lambda (x y c)
+      (let* ((lum (luminance c))
+             (above (if invert (< lum level) (> lum level))))
+        (if above
+            (rgb 255 255 255)
+            (rgb 0 0 0))))))