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/threshold.sexp

@@ -0,0 +1,9 @@
+;; Threshold effect - converts to black and white
+(require-primitives "color_ops")
+
+(define-effect threshold
+  :params (
+    (level :type int :default 128 :range [0 255])
+    (invert :type bool :default false)
+  )
+  (color_ops:threshold frame level invert))