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_vignette.sexp

@@ -0,0 +1,36 @@
+;; Vignette effect using xector operations
+;; Demonstrates α (element-wise) and β (reduction) patterns
+
+(require-primitives "xector")
+
+(define-effect xector_vignette
+  :params (
+    (strength :type float :default 0.5 :range [0 1])
+    (radius :type float :default 1.0 :range [0.5 2])
+  )
+  (let* (
+         ;; Get normalized distance from center for each pixel
+         (dist (dist-from-center frame))
+
+         ;; Calculate max distance (corner distance)
+         (max-dist (* (βmax dist) radius))
+
+         ;; Calculate brightness factor per pixel: 1 - (dist/max-dist * strength)
+         ;; Using explicit α operators
+         (factor (α- 1 (α* (α/ dist max-dist) strength)))
+
+         ;; Clamp factor to [0, 1]
+         (factor (αclamp factor 0 1))
+
+         ;; Extract channels as xectors
+         (r (red frame))
+         (g (green frame))
+         (b (blue frame))
+
+         ;; Apply factor to each channel (implicit element-wise via Xector operators)
+         (r-out (* r factor))
+         (g-out (* g factor))
+         (b-out (* b factor)))
+
+    ;; Combine back to frame
+    (rgb r-out g-out b-out)))