Added deterministic function layer

src/markitect_tool/extension/builtins.py

@@ -18,6 +18,7 @@ def builtin_extension_registry() -> ExtensionRegistry:
         _runtime_form_state_descriptor(),
         _runtime_assessment_descriptor(),
         _local_label_policy_descriptor(),
+        _document_function_descriptor(),
     ]:
         registry.register(descriptor)
     return registry
@@ -233,3 +234,34 @@ def _local_label_policy_descriptor() -> ExtensionDescriptor:
             ]
         },
     )
+
+
+def _document_function_descriptor() -> ExtensionDescriptor:
+    return ExtensionDescriptor(
+        id="document.function",
+        kind="document-function",
+        summary="Markdown-native deterministic document function registry and evaluator.",
+        capabilities=[
+            ProcessingCapability(id="document_function", kind="execute"),
+            ProcessingCapability(id="deterministic", kind="execution"),
+            ProcessingCapability(id="diagnostics", kind="emit"),
+            ProcessingCapability(id="provenance", kind="emit"),
+        ],
+        safety={
+            "network": False,
+            "filesystem": False,
+            "assisted_generation": False,
+            "external_process": False,
+        },
+        input_contract="Markdown with {{mkt:function ...}} or mkt-function fences",
+        output_contract="DocumentFunctionEvaluationResult",
+        diagnostics_namespace="document_function",
+        provenance_prefix="document_function",
+        cli={"commands": ["mkt function list", "mkt function check", "mkt function render"]},
+        docs=["docs/document-functions.md"],
+        examples=["examples/functions/basic-functions.md"],
+        metadata={
+            "execution": "deterministic-only",
+            "external_policy_services_required": False,
+        },
+    )