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/test/sexp_effects/effects/pixelsort.sexp

@@ -0,0 +1,11 @@
+;; Pixelsort effect - glitch art pixel sorting
+
+(define-effect pixelsort
+  :params (
+    (sort_by :type string :default "lightness")
+    (threshold_low :type int :default 50 :range [0 255])
+    (threshold_high :type int :default 200 :range [0 255])
+    (angle :type int :default 0 :range [0 180])
+    (reverse :type bool :default false)
+  )
+  (pixelsort frame sort_by threshold_low threshold_high angle reverse))