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/strobe.sexp

@@ -0,0 +1,16 @@
+;; Strobe effect - holds frames for choppy look
+(require-primitives "core")
+
+(define-effect strobe
+  :params (
+    (frame_rate :type int :default 12 :range [1 60])
+  )
+  (let* ((held (state-get 'held nil))
+         (held-until (state-get 'held-until 0))
+         (frame-duration (/ 1 frame_rate)))
+    (if (or (core:is-nil held) (>= t held-until))
+        (begin
+          (state-set 'held (copy frame))
+          (state-set 'held-until (+ t frame-duration))
+          frame)
+        held)))