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/film_grain.sexp

@@ -0,0 +1,19 @@
+;; Film Grain effect - adds film grain texture
+(require-primitives "core")
+
+(define-effect film_grain
+  :params (
+    (intensity :type float :default 0.2 :range [0 1])
+    (colored :type bool :default false)
+  )
+  (let ((grain-amount (* intensity 50)))
+    (map-pixels frame
+      (lambda (x y c)
+        (if colored
+            (rgb (clamp (+ (red c) (gaussian 0 grain-amount)) 0 255)
+                 (clamp (+ (green c) (gaussian 0 grain-amount)) 0 255)
+                 (clamp (+ (blue c) (gaussian 0 grain-amount)) 0 255))
+            (let ((n (gaussian 0 grain-amount)))
+              (rgb (clamp (+ (red c) n) 0 255)
+                   (clamp (+ (green c) n) 0 255)
+                   (clamp (+ (blue c) n) 0 255))))))))