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/halftone.sexp

@@ -0,0 +1,49 @@
+;; Halftone/dot effect - built from primitive xector operations
+;;
+;; Uses:
+;;   pool-frame     - downsample to cell luminances
+;;   cell-indices   - which cell each pixel belongs to
+;;   gather         - look up cell value for each pixel
+;;   local-x/y-norm - position within cell [0,1]
+;;   where          - conditional per-pixel
+
+(require-primitives "xector")
+
+(define-effect halftone
+  :params (
+    (cell-size :type int :default 12 :range [4 32] :desc "Size of halftone cells")
+    (dot-scale :type float :default 0.9 :range [0.1 1.0] :desc "Max dot radius")
+    (invert :type bool :default false :desc "Invert (white dots on black)")
+  )
+  (let* (
+         ;; Pool frame to get luminance per cell
+         (pooled (pool-frame frame cell-size))
+         (cell-lum (nth pooled 3))  ; luminance is 4th element
+
+         ;; For each output pixel, get its cell index
+         (cell-idx (cell-indices frame cell-size))
+
+         ;; Get cell luminance for each pixel
+         (pixel-lum (α/ (gather cell-lum cell-idx) 255))
+
+         ;; Position within cell, normalized to [-0.5, 0.5]
+         (lx (α- (local-x-norm frame cell-size) 0.5))
+         (ly (α- (local-y-norm frame cell-size) 0.5))
+
+         ;; Distance from cell center (0 at center, ~0.7 at corners)
+         (dist (αsqrt (α+ (α² lx) (α² ly))))
+
+         ;; Radius based on luminance (brighter = bigger dot)
+         (radius (α* (if invert (α- 1 pixel-lum) pixel-lum)
+                     (α* dot-scale 0.5)))
+
+         ;; Is this pixel inside the dot?
+         (inside (α< dist radius))
+
+         ;; Output color
+         (fg (if invert 255 0))
+         (bg (if invert 0 255))
+         (out (where inside fg bg)))
+
+    ;; Grayscale output
+    (rgb out out out)))