fix(plugin-designer): build the Field Designer's fields map as own properties and refuse what it cannot carry

.changeset/6489-designer-fields-map-keying.md

@@ -0,0 +1,32 @@
+---
+'@object-ui/plugin-designer': patch
+---
+
+The Field Designer builds an object's `fields` map by defining own properties and refuses
+the three field lists a name-keyed map cannot carry (objectui#6489). `MetadataFieldsPage`
+keyed the map by blind assignment — `nextFields[f.name] = fromDesignerField(…)` inside a
+bare `for` loop — which failed silently in three directions, all measured on the installed
+`@objectstack/spec` 17.2.0:
+
+- **A field named `__proto__` never reached the wire.** `map['__proto__'] = def` invokes the
+ prototype setter instead of creating a key, so the field vanished from the serialised PUT
+ body. `__proto__` matches `ObjectSchema.fields`' key rule `/^[a-z_][a-z0-9_]*$/`, so the
+ spec stood ready to accept the field the client had thrown away. The map is now built
+ through `Object.fromEntries`, which defines an own property.
+- **A nameless field was stored under the literal key `"undefined"`.** Measured:
+ `ObjectSchema.safeParse` with `fields: { undefined: … }` returns `success = true`, so the
+ document parsed, persisted, and had no reader anywhere. It is now refused before the
+ request.
+- **Two fields sharing a name collapsed into one entry.** A designer list carrying two
+ `amount` fields PUT a single entry, the later silently replacing the earlier. Also refused
+ before the request.
+
+Both refusals raise before `client.save`, so a refused list issues no PUT at all, and the
+message lands in the page's existing error surface naming the offending index — the caller
+is fire-and-forget (`void handleFieldsChange(next)`), so throwing past it would show the
+author nothing.
+
+This is the plugin-designer port of the refusals objectui#6240 landed in the sibling object
+writer (app-shell's `MetadataService.toFieldsMap`), down to the wording, so the two writers
+of the objectui#5761 parity family cannot drift. `fromDesignerField`'s carry-over semantics
+are untouched.