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/xector_grain.sexp

@@ -0,0 +1,34 @@
+;; Film grain effect using xector operations
+;; Demonstrates random xectors and mixing scalar/xector math
+
+(require-primitives "xector")
+
+(define-effect xector_grain
+  :params (
+    (intensity :type float :default 0.2 :range [0 1] :desc "Grain intensity")
+    (colored :type bool :default false :desc "Use colored grain")
+  )
+  (let* (
+         ;; Extract channels
+         (r (red frame))
+         (g (green frame))
+         (b (blue frame))
+
+         ;; Generate noise xector(s)
+         ;; randn-x generates normal distribution noise
+         (grain-amount (* intensity 50)))
+
+    (if colored
+        ;; Colored grain: different noise per channel
+        (let* ((nr (randn-x frame 0 grain-amount))
+               (ng (randn-x frame 0 grain-amount))
+               (nb (randn-x frame 0 grain-amount)))
+          (rgb (αclamp (α+ r nr) 0 255)
+               (αclamp (α+ g ng) 0 255)
+               (αclamp (α+ b nb) 0 255)))
+
+        ;; Monochrome grain: same noise for all channels
+        (let ((n (randn-x frame 0 grain-amount)))
+          (rgb (αclamp (α+ r n) 0 255)
+               (αclamp (α+ g n) 0 255)
+               (αclamp (α+ b n) 0 255))))))