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.json

@@ -6,9 +6,10 @@
     "execute": {"pass": 15, "fail": 0},
     "optimize": {"pass": 18, "fail": 0},
     "fed": {"pass": 15, "fail": 0},
-    "cost": {"pass": 13, "fail": 0}
+    "cost": {"pass": 13, "fail": 0},
+    "serialize": {"pass": 13, "fail": 0}
   },
-  "total_pass": 115,
+  "total_pass": 128,
   "total_fail": 0,
-  "total": 115
+  "total": 128
 }