content: nested block trees (section.sx) + 25 tests (410/410)

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

lib/content/scoreboard.json

@@ -7,6 +7,7 @@
     "meta": {"pass": 27, "fail": 0},
     "markdown": {"pass": 20, "fail": 0},
     "text": {"pass": 20, "fail": 0},
+    "section": {"pass": 25, "fail": 0},
     "validate": {"pass": 17, "fail": 0},
     "store": {"pass": 29, "fail": 0},
     "snapshot": {"pass": 20, "fail": 0},
@@ -16,7 +17,7 @@
     "md-import": {"pass": 24, "fail": 0},
     "fed": {"pass": 20, "fail": 0}
   },
-  "total_pass": 385,
+  "total_pass": 410,
   "total_fail": 0,
-  "total": 385
+  "total": 410
 }