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/pixelate.sexp

@@ -0,0 +1,13 @@
+;; Pixelate effect - creates blocky pixels
+(require-primitives "image")
+
+(define-effect pixelate
+  :params (
+    (block_size :type int :default 8 :range [2 64])
+  )
+  (let* ((w (image:width frame))
+         (h (image:height frame))
+         (small-w (max 1 (floor (/ w block_size))))
+         (small-h (max 1 (floor (/ h block_size))))
+         (small (image:resize frame small-w small-h "area")))
+    (image:resize small w h "nearest")))