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/posterize.sexp

@@ -0,0 +1,8 @@
+;; Posterize effect - reduces color levels
+(require-primitives "color_ops")
+
+(define-effect posterize
+  :params (
+    (levels :type int :default 8 :range [2 32])
+  )
+  (color_ops:posterize frame levels))