Data grid 2 settings written by their widget key are lost on a describe → exec round trip, and check without a project calls them "silently dropped" #1573

Repro (v0.27.0 and current main; Mendix 11.12.5)

create persistent entity Shop.GridPref (Config: String(unlimited));
create page Shop.Ord_Local ( Title: 'Orders', Layout: Atlas_Core.Atlas_Default ) {
  datagrid dgLocal (DataSource: database Shop.Ord, refreshInterval: 30, columnsHidable: false,
                    configurationStorageType: localStorage, storeFiltersInPersonalization: true) {
    column (Attribute: Code) { textfilter tf }
  }
};
create page Shop.Ord_Attr ( Title: 'Orders', Layout: Atlas_Core.Atlas_Default, Params: { $Pref: Shop.GridPref } ) {
  dataview dvPref (DataSource: $Pref) {
    datagrid dgAttr (DataSource: database Shop.Ord, configurationStorageType: attribute, configurationAttribute: Config) {
      column (Attribute: Code) { textfilter tf2 }
    }
  }
};

check -p passes, and it even applies the editor-config rules: columnsHidable: false together with a column Hidable: no is refused (MDL-WIDGET10). exec stores every value, and mx check reports 0 errors. Two things go wrong after that.

1. describe page prints none of these settings. It prints datagrid dgLocal (DataSource: database from Shop.Ord). A describe → drop → exec round trip therefore resets them to the defaults. On a five-grid test page set, 0 of 6 pages came back byte-identical. The same is true for Gallery's settings, a column's own settings and the emptyPlaceholder slot. A Studio Pro marketplace page (Administration.Account_Overview) also loses onClickTrigger: 'double'.
2. check without a project (mxcli check pers.mdl, as make check-mdl runs it) warns, for each of these keys, that the property "is not recognized and will be silently dropped on write" (MDL-WIDGET07), although exec stores it. For an expression-typed key, such as rowClass or a column's columnClass, it is an MDL-WIDGET42 error, so check exits 1. The cause is that DATAGRID has no embedded definition, since its .def.json is generated from the project's package. Without a project, the keyword falls back to the static-widget vocabulary.

These settings include the personalization ones (where to store column, sort and filter state), which is a common thing to want to set from MDL.

Also found (not fixed in the PRs below)

• By-key selection is not stored. itemSelection: Single by key is stored as PrimitiveValue 'Single' with Selection: None, so the grid has no selection. Selection: Single works. This looks like the explicit-key pass treating a selection-typed key as a primitive.
• Other widgets lose by-key settings on describe the same way. Combobox (clearable, showFooter), textfilter (adjustable, delay), dropdownfilter (filterable) and image (isBackgroundImage) all accept them in check and exec, and describe drops them. Each has its own describe branch.

I have two PRs ready, one per gap, and will link them.