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.md

@@ -11,6 +11,7 @@ _Generated by `lib/content/conformance.sh`_
 | meta | 27 | 0 | 27 |
 | markdown | 20 | 0 | 20 |
 | text | 20 | 0 | 20 |
+| section | 25 | 0 | 25 |
 | validate | 17 | 0 | 17 |
 | store | 29 | 0 | 29 |
 | snapshot | 20 | 0 | 20 |
@@ -19,4 +20,4 @@ _Generated by `lib/content/conformance.sh`_
 | sync | 14 | 0 | 14 |
 | md-import | 24 | 0 | 24 |
 | fed | 20 | 0 | 20 |
-| **Total** | **385** | **0** | **385** |
+| **Total** | **410** | **0** | **410** |