content: deep tree editing (tree-edit.sx) + 17 tests (551/551)

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

lib/content/scoreboard.json

@@ -11,6 +11,7 @@
     "text": {"pass": 20, "fail": 0},
     "section": {"pass": 25, "fail": 0},
     "compose": {"pass": 17, "fail": 0},
+    "tree-edit": {"pass": 17, "fail": 0},
     "stats": {"pass": 17, "fail": 0},
     "table": {"pass": 15, "fail": 0},
     "data": {"pass": 21, "fail": 0},
@@ -25,7 +26,7 @@
     "md-doc": {"pass": 12, "fail": 0},
     "fed": {"pass": 20, "fail": 0}
   },
-  "total_pass": 534,
+  "total_pass": 551,
   "total_fail": 0,
-  "total": 534
+  "total": 551
 }