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.md

@@ -0,0 +1,8 @@
+# artdag Conformance Scoreboard
+
+_Generated by `lib/artdag/conformance.sh`_
+
+| Suite | Pass | Fail | Total |
+|-------|-----:|-----:|------:|
+| dag | 20 | 0 | 20 |
+| **Total** | **20** | **0** | **20** |