Add render reference asset manifest contract

docs/internal-extension-framework.md

@@ -42,6 +42,7 @@ framework organizes how Markitect itself exposes and composes capabilities.
 | `source-adapter` | EPUB3/PDF/DOCX adapters in external packages | source asset in, normalized Markdown out |
 | `cli-group` | cache, backend, ref, class | command descriptors or registration hook |
 | `render-export` | Quarkdown/export adapters | Markdown source in, rendered/exported artifact descriptor out |
+| `render-reference-contract` | render units, cross-references, TOC, asset manifests | passive manifest in, renderer-planning metadata out |
 | `document-function` | future function layer | function call in, typed document value out |
 
 ## Canonical Lifecycle
@@ -172,8 +173,10 @@ Each module exposes one or more descriptors plus a registration function. The
 root registry can be assembled explicitly at import time or by a small internal
 discovery list. Source adapters are the first external package-discovery slice
 and use the `markitect_tool.source_adapters` entry point group defined in
-`docs/source-adapter-contract.md`; other extension kinds can adopt package
-entry points later if they become a real requirement.
+`docs/source-adapter-contract.md`. Render/export adapters use
+`markitect_tool.render_export_adapters` and keep concrete renderer execution in
+external packages. Render reference and asset manifests remain built-in passive
+contracts; they do not need adapter discovery.
 
 See `docs/extension-authoring.md` for the extension authoring checklist and
 descriptor template.