artdag: Phase 1 DAG model + structural content addressing + 20 tests

Content-addressed node = {:op :inputs :params :commutative}; content-id is a
deterministic canonical serialization (sorted param keys; commutative ops sort
inputs). artdag/build validates dangling/cycles, topo-sorts, dedups identical
subgraphs to one id shared across DAGs. conformance.sh + scoreboard (dag 20/20).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/artdag/scoreboard.json

@@ -0,0 +1,8 @@
+{
+  "suites": {
+    "dag": {"pass": 20, "fail": 0}
+  },
+  "total_pass": 20,
+  "total_fail": 0,
+  "total": 20
+}