artdag: DAG wire serialization — portable record form + integrity + 13 tests

serialize.sx emits a topo-ordered (id op inputs params commutative) record list
that survives write/read (string-keyed node dicts do not; empty inputs read back
as nil and are normalized). wire->dag reconstructs a runnable dag by content-id;
wire-verify recomputes ids to reject tampering. dag->string/string->dag for text
transport. serialize 13/13, total 128/128.

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

lib/artdag/scoreboard.md

@@ -11,4 +11,5 @@ _Generated by `lib/artdag/conformance.sh`_
 | optimize | 18 | 0 | 18 |
 | fed | 15 | 0 | 15 |
 | cost | 13 | 0 | 13 |
-| **Total** | **115** | **0** | **115** |
+| serialize | 13 | 0 | 13 |
+| **Total** | **128** | **0** | **128** |