content: anchored-heading render (anchor.sx) + 6 tests (621/621)

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

lib/content/scoreboard.json

@@ -15,6 +15,7 @@
     "clone": {"pass": 10, "fail": 0},
     "query": {"pass": 13, "fail": 0},
     "toc": {"pass": 8, "fail": 0},
+    "anchor": {"pass": 6, "fail": 0},
     "transform": {"pass": 12, "fail": 0},
     "normalize": {"pass": 11, "fail": 0},
     "find-replace": {"pass": 10, "fail": 0},
@@ -32,7 +33,7 @@
     "md-doc": {"pass": 12, "fail": 0},
     "fed": {"pass": 20, "fail": 0}
   },
-  "total_pass": 615,
+  "total_pass": 621,
   "total_fail": 0,
-  "total": 615
+  "total": 621
 }