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/bloom.sexp

@@ -0,0 +1,14 @@
+;; Bloom effect - glow on bright areas
+;; @param intensity float [0, 2] default 0.5
+;; @param threshold int [0, 255] default 200
+;; @param radius int [1, 50] default 15
+
+(define-effect bloom
+  ((intensity 0.5) (threshold 200) (radius 15))
+  (let* ((bright (map-pixels frame
+                   (lambda (x y c)
+                     (if (> (luminance c) threshold)
+                         c
+                         (rgb 0 0 0)))))
+         (blurred (blur bright radius)))
+    (blend-mode frame blurred "add")))