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/mosaic.sexp

@@ -0,0 +1,30 @@
+;; Mosaic effect - built from primitive xector operations
+;;
+;; Uses:
+;;   pool-frame  - downsample to cell averages
+;;   cell-indices - which cell each pixel belongs to
+;;   gather       - look up cell value for each pixel
+
+(require-primitives "xector")
+
+(define-effect mosaic
+  :params (
+    (cell-size :type int :default 16 :range [4 64] :desc "Size of mosaic cells")
+  )
+  (let* (
+         ;; Pool frame to get average color per cell (returns r,g,b,lum xectors)
+         (pooled (pool-frame frame cell-size))
+         (cell-r (nth pooled 0))
+         (cell-g (nth pooled 1))
+         (cell-b (nth pooled 2))
+
+         ;; For each output pixel, get its cell index
+         (cell-idx (cell-indices frame cell-size))
+
+         ;; Gather: look up cell color for each pixel
+         (out-r (gather cell-r cell-idx))
+         (out-g (gather cell-g cell-idx))
+         (out-b (gather cell-b cell-idx)))
+
+    ;; Reconstruct frame
+    (rgb out-r out-g out-b)))