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_threshold.sexp

@@ -0,0 +1,27 @@
+;; Threshold effect using xector operations
+;; Demonstrates where (conditional select) and β (reduction) for normalization
+
+(require-primitives "xector")
+
+(define-effect xector_threshold
+  :params (
+    (threshold :type float :default 0.5 :range [0 1] :desc "Brightness threshold (0-1)")
+    (invert :type bool :default false :desc "Invert the threshold")
+  )
+  (let* (
+         ;; Get grayscale luminance as xector
+         (luma (gray frame))
+
+         ;; Normalize to 0-1 range
+         (luma-norm (α/ luma 255))
+
+         ;; Create boolean mask: pixels above threshold
+         (mask (if invert
+                   (α< luma-norm threshold)
+                   (α>= luma-norm threshold)))
+
+         ;; Use where to select: white (255) if above threshold, black (0) if below
+         (out (where mask 255 0)))
+
+    ;; Output as grayscale (same value for R, G, B)
+    (rgb out out out)))