DATAGRID column + DROPDOWNFILTER cannot bind to an association (CE1613); Engalar additionally drops the filter silently

[MendixMau]

Environment: mxcli (RnD 504aec67) and Engalar/mxcli (26f2866), Mendix 11.12.0, macOS host, mxbuild via Studio Pro 11.12.0 (mxcli docker check).

Steps to reproduce:

create module ZKT38;
/
create entity ZKT38.Customer ( Name: String );
create entity ZKT38.Order ( Number: String );
create association ZKT38.Order_Customer from ZKT38.Order to ZKT38.Customer;
/
create or replace page ZKT38.OrderList
( Title: 'Orders', Layout: Atlas_Core.Atlas_Default )
{
  datagrid dgOrders (datasource: database ZKT38.Order) {
    column colCustomer (attribute: Order_Customer, caption: 'Customer') {
      dropdownfilter ddfCustomer
    }
  }
}
/
describe page ZKT38.OrderList;

Run with exec then mxcli docker check (mxbuild).

Expected behavior: Studio Pro supports filtering a datagrid column by an association
(reference-selector-backed column with a dropdown filter over the related entity). MDL should be
able to express this — either by allowing column (attribute: <AssociationName>, ...) to
resolve as an association path, and/or by giving dropdownfilter its own association-mode
properties (analogous to combobox's Association: + datasource: + CaptionAttribute:)
instead of only ever inheriting FirstAttribute from the parent column in linked mode.

Actual behavior:

• Both forks: mx check fails with [CE1613] "The selected attribute 'ZKT38.Order.Order_Customer' no longer exists." at Columns (1/1) of data grid 2 'dgOrders' — the association is written into the column's Attribute property as if it were a plain attribute path, which Studio Pro rejects.
• Engalar additionally silently drops the nested dropdownfilter ddfCustomer widget entirely from the column during exec/round-trip — it is absent from describe page output, with no warning or error.
• Neither fork's widget definitions (dropdownfilter.def.json) expose a refEntity/refOptions/Pages$MicroflowSource-shaped association mode; the only supported mode is attrChoice: linked against the parent column's own (non-association) attribute.

Not fixed on either fork as of this test. No workaround via plain MDL exists; the only known workaround is a live MCP pg_patch_page call with an explicit refOptions/refEntity/Pages$MicroflowSource shape (not exercised in this test — requires live MCP + Studio Pro).