Add render reference asset manifest contract

docs/render-export-adapters.md

@@ -56,6 +56,12 @@ identity, options, and local policy flags.
 - source-to-render provenance
 - metadata such as whether an external renderer was invoked
 
+`RenderExportRequest` may also carry a passive render reference manifest. That
+manifest is defined in `docs/render-reference-asset-manifest.md` and lets core
+Markitect pass unit identities, cross-reference intent, TOC planning, source
+maps, and asset descriptors to a renderer without assigning final numbering or
+copying assets.
+
 Artifacts are descriptors, not durable storage records. Real renderer packages
 may write files, but core Markitect only models the result.