Add Quarkdown render adapter boundary

docs/adapter-boundary.md

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+# Markitect Quarkdown Adapter Boundary
+
+`markitect-quarkdown` provides the concrete `render.quarkdown` adapter for the
+Markitect render/export contract. Core `markitect-tool` owns the passive
+request/result/artifact/provenance contracts; this repository owns Quarkdown
+runtime behavior.
+
+## Adapter Contract
+
+The adapter supports:
+
+- `inspect-profile`
+- `export-source`
+- `render-artifact`
+
+`inspect-profile` and `export-source` do not invoke Quarkdown. `render-artifact`
+requires a Quarkdown CLI plus runtime dependencies.
+
+The adapter descriptor declares filesystem writes, external process execution,
+native renderer dependency, and permission-controlled network behavior. Callers
+can block render execution through Markitect request policy:
+
+```python
+RenderExportRequest(
+    source="# Demo",
+    operation="render-artifact",
+    profile="pdf",
+    policy={"external_process": False},
+)
+```
+
+## Execution Plan
+
+`build_quarkdown_execution_plan` produces an inspectable command plan with:
+
+- Quarkdown command path
+- source file path
+- output directory
+- expected artifact path
+- Markitect profile
+- Quarkdown document type
+- output format
+- permission flags
+- runtime dependency notes
+
+The default output directory is `quarkdown-output`. The default permission
+allow-list is `project-read`, and network access is denied by default.
+
+## Structured Failure
+
+The adapter returns `RenderExportResult` diagnostics instead of raising for
+expected render failures:
+
+- `render.quarkdown.runtime_missing`
+- `render.quarkdown.execution_failed`
+- `render.quarkdown.artifact_missing`
+- `render.quarkdown.artifact_extension`
+- `render.quarkdown.artifact_empty`
+- `render.quarkdown.capability_blocked`
+
+## Artifact Validation
+
+`validate_quarkdown_artifact` validates that the expected artifact exists, has
+the expected extension, is non-empty, and receives a sha256 digest. The result
+is a Markitect `RenderArtifact` with source-to-artifact provenance generated by
+the adapter.