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/flip.sexp

@@ -0,0 +1,16 @@
+;; Flip effect - flips image horizontally or vertically
+(require-primitives "geometry")
+
+(define-effect flip
+  :params (
+    (horizontal :type bool :default true)
+    (vertical :type bool :default false)
+  )
+  (let ((result frame))
+    (if horizontal
+        (set! result (geometry:flip-img result "horizontal"))
+        nil)
+    (if vertical
+        (set! result (geometry:flip-img result "vertical"))
+        nil)
+    result))