Add ascii_dual_blend recipe and fix blend/layer effects

- Add ascii_dual_blend.sexp: blends two ASCII-processed videos synced to audio
- Fix blend.sexp: add require-primitives, fix params syntax
- Fix layer.sexp: add require-primitives
- Use consistent (effect blend ...) syntax instead of special form

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

sexp_effects/effects/layer.sexp

@@ -2,6 +2,8 @@
 ;; Multi-input effect: uses frame-a (background) and frame-b (overlay)
 ;; Params: x, y (position), opacity (0-1), mode (blend mode)
 
+(require-primitives "image" "blending")
+
 (define-effect layer
   :params (
     (x :type int :default 0)