Incorporate art-dag-mono repo into artdag/ subfolder

Merges full history from art-dag/mono.git into the monorepo
under the artdag/ directory. Contains: core (DAG engine),
l1 (Celery rendering server), l2 (ActivityPub registry),
common (shared templates/middleware), client (CLI), test (e2e).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

git-subtree-dir: artdag
git-subtree-mainline: 1a179de547
git-subtree-split: 4c2e716558

artdag/l1/sexp_effects/effects/blend.sexp

@@ -0,0 +1,31 @@
+;; Blend effect - combines two video frames
+;; Streaming-compatible: frame is background, overlay is second frame
+;; Usage: (blend background overlay :opacity 0.5 :mode "alpha")
+;;
+;; Params:
+;;   mode - blend mode (add, multiply, screen, overlay, difference, lighten, darken, alpha)
+;;   opacity - blend amount (0-1)
+
+(require-primitives "image" "blending" "core")
+
+(define-effect blend
+  :params (
+    (overlay :type frame :default nil)
+    (mode :type string :default "alpha")
+    (opacity :type float :default 0.5)
+  )
+  (if (core:is-nil overlay)
+    frame
+    (let [a frame
+          b overlay
+          a-h (image:height a)
+          a-w (image:width a)
+          b-h (image:height b)
+          b-w (image:width b)
+          ;; Resize b to match a if needed
+          b-sized (if (and (= a-w b-w) (= a-h b-h))
+                    b
+                    (image:resize b a-w a-h "linear"))]
+      (if (= mode "alpha")
+        (blending:blend-images a b-sized opacity)
+        (blending:blend-images a (blending:blend-mode a b-sized mode) opacity)))))