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/common/artdag_common/__init__.py

@@ -0,0 +1,18 @@
+"""
+Art-DAG Common Library
+
+Shared components for L1 (celery) and L2 (activity-pub) servers.
+"""
+
+from .constants import NODE_COLORS, TAILWIND_CDN, HTMX_CDN, CYTOSCAPE_CDN
+from .rendering import create_jinja_env, render, render_fragment
+
+__all__ = [
+    "NODE_COLORS",
+    "TAILWIND_CDN",
+    "HTMX_CDN",
+    "CYTOSCAPE_CDN",
+    "create_jinja_env",
+    "render",
+    "render_fragment",
+]