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/echo.sexp

@@ -0,0 +1,19 @@
+;; Echo effect - motion trails using frame buffer
+(require-primitives "blending")
+
+(define-effect echo
+  :params (
+    (num_echoes :type int :default 4 :range [1 20])
+    (decay :type float :default 0.5 :range [0 1])
+  )
+  (let* ((buffer (state-get "buffer" (list)))
+         (new-buffer (take (cons frame buffer) (+ num_echoes 1))))
+    (begin
+      (state-set "buffer" new-buffer)
+      ;; Blend frames with decay
+      (if (< (length new-buffer) 2)
+          frame
+          (let ((result (copy frame)))
+            ;; Simple blend of first two frames for now
+            ;; Full version would fold over all frames
+            (blending:blend-images frame (nth new-buffer 1) (* decay 0.5)))))))