Markitect schema-validation integration use case and fixture for Markdown proxy documents

docs/architecture-blueprint.md

@@ -206,6 +206,11 @@ Adapter rules:
   checks, and context-package interoperability. Engine domain code must not
   import it directly; adapter code should persist serializable Markitect
   outputs as adapter provenance or representation metadata.
+- Markdown proxy documents are allowed as adapter projections for managed
+  assets. They can make every asset inspectable and contract-checkable through
+  Markitect where useful, but they are not the canonical engine identity or
+  storage model. The canonical layer remains asset, representation, metadata,
+  lifecycle, policy, lineage, and audit state.
 - `llm-connect` or equivalent is an adapter for LLM providers.
 - `phase-memory` is an adjacent memory runtime; this engine may exchange opaque
   memory references or context packages but should not implement memory phases.