content: document composition (compose.sx) + 17 tests (502/502)

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

lib/content/scoreboard.md

@@ -14,6 +14,7 @@ _Generated by `lib/content/conformance.sh`_
 | markdown | 20 | 0 | 20 |
 | text | 20 | 0 | 20 |
 | section | 25 | 0 | 25 |
+| compose | 17 | 0 | 17 |
 | stats | 17 | 0 | 17 |
 | table | 15 | 0 | 15 |
 | validate | 23 | 0 | 23 |
@@ -25,4 +26,4 @@ _Generated by `lib/content/conformance.sh`_
 | md-import | 38 | 0 | 38 |
 | md-doc | 12 | 0 | 12 |
 | fed | 20 | 0 | 20 |
-| **Total** | **485** | **0** | **485** |
+| **Total** | **502** | **0** | **502** |