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/trails.sexp

@@ -0,0 +1,20 @@
+;; Trails effect - persistent motion trails
+(require-primitives "image" "blending")
+
+(define-effect trails
+  :params (
+    (persistence :type float :default 0.8 :range [0 0.99])
+  )
+  (let* ((buffer (state-get "buffer" nil))
+         (current frame))
+    (if (= buffer nil)
+        (begin
+          (state-set "buffer" (copy frame))
+          frame)
+        (let* ((faded (blending:blend-images buffer
+                                    (make-image (image:width frame) (image:height frame) (list 0 0 0))
+                                    (- 1 persistence)))
+               (result (blending:blend-mode faded current "lighten")))
+          (begin
+            (state-set "buffer" result)
+            result)))))