content: document flatten (flatten.sx) + 10 tests (645/645)

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

lib/content/scoreboard.json

@@ -17,6 +17,7 @@
     "toc": {"pass": 8, "fail": 0},
     "anchor": {"pass": 6, "fail": 0},
     "outline": {"pass": 14, "fail": 0},
+    "flatten": {"pass": 10, "fail": 0},
     "transform": {"pass": 12, "fail": 0},
     "normalize": {"pass": 11, "fail": 0},
     "find-replace": {"pass": 10, "fail": 0},
@@ -34,7 +35,7 @@
     "md-doc": {"pass": 12, "fail": 0},
     "fed": {"pass": 20, "fail": 0}
   },
-  "total_pass": 635,
+  "total_pass": 645,
   "total_fail": 0,
-  "total": 635
+  "total": 645
 }