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/vignette.sexp

@@ -0,0 +1,23 @@
+;; Vignette effect - darkens corners
+(require-primitives "image")
+
+(define-effect vignette
+  :params (
+    (strength :type float :default 0.5 :range [0 1])
+    (radius :type int :default 1 :range [0.5 2])
+  )
+  (let* ((w (image:width frame))
+         (h (image:height frame))
+         (cx (/ w 2))
+         (cy (/ h 2))
+         (max-dist (* (sqrt (+ (* cx cx) (* cy cy))) radius)))
+    (map-pixels frame
+      (lambda (x y c)
+        (let* ((dx (- x cx))
+               (dy (- y cy))
+               (dist (sqrt (+ (* dx dx) (* dy dy))))
+               (factor (- 1 (* (/ dist max-dist) strength)))
+               (factor (clamp factor 0 1)))
+          (rgb (* (red c) factor)
+               (* (green c) factor)
+               (* (blue c) factor)))))))