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/ascii_art.sexp

@@ -0,0 +1,17 @@
+;; ASCII Art effect - converts image to ASCII characters
+(require-primitives "ascii")
+
+(define-effect ascii_art
+  :params (
+    (char_size :type int :default 8 :range [4 32])
+    (alphabet :type string :default "standard")
+    (color_mode :type string :default "color" :desc "color, mono, invert, or any color name/hex")
+    (background_color :type string :default "black" :desc "background color name/hex")
+    (invert_colors :type int :default 0 :desc "swap foreground and background colors")
+    (contrast :type float :default 1.5 :range [1 3])
+  )
+  (let* ((sample (cell-sample frame char_size))
+         (colors (nth sample 0))
+         (luminances (nth sample 1))
+         (chars (luminance-to-chars luminances alphabet contrast)))
+    (render-char-grid frame chars colors char_size color_mode background_color invert_colors)))