content: document statistics (stats.sx) + 17 tests (433/433)

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

lib/content/scoreboard.json

@@ -8,6 +8,7 @@
     "markdown": {"pass": 20, "fail": 0},
     "text": {"pass": 20, "fail": 0},
     "section": {"pass": 25, "fail": 0},
+    "stats": {"pass": 17, "fail": 0},
     "validate": {"pass": 23, "fail": 0},
     "store": {"pass": 29, "fail": 0},
     "snapshot": {"pass": 20, "fail": 0},
@@ -17,7 +18,7 @@
     "md-import": {"pass": 24, "fail": 0},
     "fed": {"pass": 20, "fail": 0}
   },
-  "total_pass": 416,
+  "total_pass": 433,
   "total_fail": 0,
-  "total": 416
+  "total": 433
 }