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/layer.sexp

@@ -0,0 +1,21 @@
+;; Layer effect - composite frame-b over frame-a at position
+;; Multi-input effect: uses frame-a (background) and frame-b (overlay)
+;; Params: x, y (position), opacity (0-1), mode (blend mode)
+
+(define-effect layer
+  ((x 0) (y 0) (opacity 1.0) (mode "alpha"))
+  (let [bg (copy frame-a)
+        fg frame-b
+        ;; Resize fg if needed to fit
+        fg-w (width fg)
+        fg-h (height fg)]
+    (if (= opacity 1.0)
+      ;; Simple paste
+      (paste bg fg x y)
+      ;; Blend with opacity
+      (let [blended (if (= mode "alpha")
+                      (blend-images (crop bg x y fg-w fg-h) fg opacity)
+                      (blend-images (crop bg x y fg-w fg-h)
+                                   (blend-mode (crop bg x y fg-w fg-h) fg mode)
+                                   opacity))]
+        (paste bg blended x y)))))