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.md

@@ -28,6 +28,7 @@ _Generated by `lib/content/conformance.sh`_
 | find-replace | 10 | 0 | 10 |
 | stats | 17 | 0 | 17 |
 | summary | 14 | 0 | 14 |
+| index | 13 | 0 | 13 |
 | table | 15 | 0 | 15 |
 | callout | 12 | 0 | 12 |
 | media | 15 | 0 | 15 |
@@ -42,4 +43,4 @@ _Generated by `lib/content/conformance.sh`_
 | md-import | 38 | 0 | 38 |
 | md-doc | 12 | 0 | 12 |
 | fed | 20 | 0 | 20 |
-| **Total** | **697** | **0** | **697** |
+| **Total** | **710** | **0** | **710** |