content: multi-doc index + tag filtering (index.sx) + 13 tests (710/710)

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

lib/content/scoreboard.json

@@ -24,6 +24,7 @@
     "find-replace": {"pass": 10, "fail": 0},
     "stats": {"pass": 17, "fail": 0},
     "summary": {"pass": 14, "fail": 0},
+    "index": {"pass": 13, "fail": 0},
     "table": {"pass": 15, "fail": 0},
     "callout": {"pass": 12, "fail": 0},
     "media": {"pass": 15, "fail": 0},
@@ -39,7 +40,7 @@
     "md-doc": {"pass": 12, "fail": 0},
     "fed": {"pass": 20, "fail": 0}
   },
-  "total_pass": 697,
+  "total_pass": 710,
   "total_fail": 0,
-  "total": 697
+  "total": 710
 }