feat(python): enforce ERR_RESERVED_ATTR in parser (ADR-0006 §D1)

Adds inline detection of @-prefixed reserved structural keys (name, package,
extends, abstract, overlay, isArray, children, value) during parse. Any
such key emits ERR_RESERVED_ATTR with the offending node and key in the
message — matches the TS parser-core behaviour.

The reserved-key tuple lives in parser.py and is built from the existing
shared.structural KEY_* constants — single source of truth.

Migrates two codegen golden fixtures (nested-array, scalars) from the
deprecated @isarray attribute form to the bare isArray structural key per
ADR-0006 §D1.

Ledger: un-gates error-reserved-word-as-attr.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: dmealing

server/python/src/metaobjects/parser.py

@@ -21,6 +21,19 @@
     KEY_VALUE,
 )
 
+# Reserved structural body keys — authoring any of these with the @-prefix is a
+# hard ERR_RESERVED_ATTR (ADR-0007). Detected inline as each @-key is processed.
+_RESERVED_STRUCTURAL_KEYS: frozenset[str] = frozenset({
+    KEY_NAME,
+    KEY_PACKAGE,
+    KEY_EXTENDS,
+    KEY_ABSTRACT,
+    KEY_OVERLAY,
+    KEY_IS_ARRAY,
+    KEY_CHILDREN,
+    KEY_VALUE,
+})
+
 
 @dataclass
 class ParseResult:
@@ -99,6 +112,18 @@ def _build(
     for key, value in body_dict.items():
         if key.startswith(ATTR_PREFIX):
             attr_name = key[len(ATTR_PREFIX):]
+            # ADR-0007: @-prefixing a reserved structural body key is invalid.
+            # Detected inline as each @-attr key is processed (matches TS parser-core).
+            if attr_name in _RESERVED_STRUCTURAL_KEYS:
+                result.errors.append(
+                    MetaError(
+                        f"node '{wrapper}' uses reserved structural key '{attr_name}' "
+                        f"with @-prefix; bare '{attr_name}' is the canonical form",
+                        ErrorCode.ERR_RESERVED_ATTR,
+                        source,
+                    )
+                )
+                continue
             schema = registry.attr_schema(type_, sub_type, attr_name)
             node.set_attr(attr_name, value, sub_type=schema.value_type if schema else None)
 

server/python/tests/codegen/golden/nested-array/meta.json

@@ -4,6 +4,6 @@
   ]}},
   { "object.value": { "name": "AuthorBrief", "children": [
     { "field.string": { "name": "displayName", "@required": true } },
-    { "field.object": { "name": "posts", "@objectRef": "PostBrief", "@isArray": true } }
+    { "field.object": { "name": "posts", "@objectRef": "PostBrief", "isArray": true } }
   ]}}
 ]}}

server/python/tests/codegen/golden/scalars/meta.json

@@ -6,6 +6,6 @@
     { "field.timestamp": { "name": "generatedAt", "@required": true } },
     { "field.decimal": { "name": "total", "@required": true } },
     { "field.currency": { "name": "revenue", "@required": true } },
-    { "field.object": { "name": "metrics", "@objectRef": "Metric", "@isArray": true } }
+    { "field.object": { "name": "metrics", "@objectRef": "Metric", "isArray": true } }
   ]}}
 ]}}

server/python/tests/conformance/conformance-expected-failures.json

@@ -2,7 +2,6 @@
   "fixtures": [
     "error-field-object-storage-flattened-array",
     "error-field-object-storage-no-object-ref",
-    "error-reserved-word-as-attr",
     "error-template-payload-ref-unresolved",
     "error-template-prompt-missing-payload-ref",
     "error-template-required-slot-missing",