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

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

lib/content/scoreboard.json

@@ -10,6 +10,7 @@
     "markdown": {"pass": 20, "fail": 0},
     "text": {"pass": 20, "fail": 0},
     "section": {"pass": 25, "fail": 0},
+    "compose": {"pass": 17, "fail": 0},
     "stats": {"pass": 17, "fail": 0},
     "table": {"pass": 15, "fail": 0},
     "validate": {"pass": 23, "fail": 0},
@@ -22,7 +23,7 @@
     "md-doc": {"pass": 12, "fail": 0},
     "fed": {"pass": 20, "fail": 0}
   },
-  "total_pass": 485,
+  "total_pass": 502,
   "total_fail": 0,
-  "total": 485
+  "total": 502
 }