content: portable data serialization (data.sx) + 21 tests (523/523)

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

lib/content/scoreboard.json

@@ -13,6 +13,7 @@
     "compose": {"pass": 17, "fail": 0},
     "stats": {"pass": 17, "fail": 0},
     "table": {"pass": 15, "fail": 0},
+    "data": {"pass": 21, "fail": 0},
     "validate": {"pass": 23, "fail": 0},
     "store": {"pass": 29, "fail": 0},
     "snapshot": {"pass": 20, "fail": 0},
@@ -23,7 +24,7 @@
     "md-doc": {"pass": 12, "fail": 0},
     "fed": {"pass": 20, "fail": 0}
   },
-  "total_pass": 502,
+  "total_pass": 523,
   "total_fail": 0,
-  "total": 502
+  "total": 523
 }