content: Markdown render mode (asMarkdown) + 20 tests (263/263)

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

lib/content/scoreboard.json

@@ -4,12 +4,13 @@
     "doc": {"pass": 40, "fail": 0},
     "render": {"pass": 42, "fail": 0},
     "api": {"pass": 26, "fail": 0},
+    "markdown": {"pass": 20, "fail": 0},
     "store": {"pass": 29, "fail": 0},
     "crdt": {"pass": 34, "fail": 0},
     "sync": {"pass": 14, "fail": 0},
     "fed": {"pass": 20, "fail": 0}
   },
-  "total_pass": 243,
+  "total_pass": 263,
   "total_fail": 0,
-  "total": 243
+  "total": 263
 }