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/trails.sexp

@@ -0,0 +1,18 @@
+;; Trails effect - persistent motion trails
+;; @param persistence float [0, 0.99] default 0.8
+
+(define-effect trails
+  ((persistence 0.8))
+  (let* ((buffer (state-get 'buffer nil))
+         (current frame))
+    (if (= buffer nil)
+        (begin
+          (state-set 'buffer (copy frame))
+          frame)
+        (let* ((faded (blend-images buffer
+                                    (make-image (width frame) (height frame) (list 0 0 0))
+                                    (- 1 persistence)))
+               (result (blend-mode faded current "lighten")))
+          (begin
+            (state-set 'buffer result)
+            result)))))