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

docs/markitect-tool-reuse-boundary.md

@@ -21,6 +21,13 @@ state should persist serializable envelopes, source references, digests,
 lineage, policy decisions, and audit events rather than storing Markitect
 runtime objects as canonical engine entities.
 
+Markdown proxy documents are a supported adapter pattern. The engine may create
+or store Markdown representations that proxy non-Markdown assets so Markitect
+selectors, contracts, document schemas, functions, and workflows can operate on
+them. Those proxies are representations of governed assets, not replacements
+for engine-owned asset identity, metadata, lifecycle, policy, lineage, or audit
+state.
+
 Required integration behavior is captured in
 `docs/markitect-tool-integration-usecases.md` and exercised by
 `tests/test_markitect_tool_contract.py`. These tests are allowed to skip when
@@ -35,6 +42,7 @@ stability checks for the boundary when the `markdown` extra is installed.
 | Document-level selectors and extraction | `markitect_tool.query`, `docs/query-extraction.md` | Use for markdown source extraction and context package creation. Engine query should operate over persisted artifacts and relationships. |
 | Deterministic transforms, composition, and includes | `markitect_tool.ops.engine`, `docs/transform-compose-include.md` | Treat as external operations invoked by workflows. Store operation provenance and derived artifacts in the engine. |
 | Contract checks, runtime context, forms, and assessments | `markitect_tool.contract.*`, `markitect_tool.runtime.*`, `docs/runtime-context-forms-assessments.md` | Use as validation/assessment step adapters. Engine owns run state and audit trail. |
+| Markdown document schema validation | `markitect_tool.schema.*` | Use for Markdown document/proxy validation. Engine-owned asset metadata validation stays in the registry layer. |
 | Backend manifests, local snapshots, FTS, and query adapters | `markitect_tool.backend.*`, `docs/backend-fabric.md` | Reuse snapshot identity and local index concepts. Engine storage remains separate and cross-format. |
 | Agent working memory context packages | `markitect_tool.memory.engine`, `docs/agent-working-memory.md` | Reuse as a portable context-package format for markdown-backed context. Engine should provide durable context registries across formats. |
 | Workflow definition syntax and markdown-centered step kinds | `markitect_tool.workflow.*`, `docs/workflow-definition-standard.md` | Reuse where workflows consume markdown inputs. Engine workflows should generalize to artifact collections, external tools, and service operations. |
@@ -44,7 +52,8 @@ stability checks for the boundary when the `markdown` extra is installed.
 ## Adapter Ownership Rules
 
 - Markdown ingestion adapters may call `parse_markdown`, `parse_markdown_file`,
-  `query_document`, `extract_document`, and `snapshot_identity_for_file`.
+  `query_document`, `extract_document`, `validate_document`,
+  `validate_markdown_file`, and `snapshot_identity_for_file`.
 - Markdown transformation adapters may call `transform_markdown`,
   `compose_files`, `resolve_includes`, Markitect contract checks, document
   functions, templates, and workflow helpers.