Document a data-heavy widget recipe without project-local React/Mendix FFI

[GG-O-BP]

Context

Once Mendraw and Glendix provide typed datasource snapshots, editable-value conversion, keyed React hosting, module initialization, and browser file/environment helpers, downstream projects need a clear recipe for data-heavy widgets that use those capabilities instead of adding project-local React/Mendix FFI.

MendixWidgetGleam is the reference widget and scaffolding CLI, so it is the right place to propagate that architecture.

Proposal

After the upstream capabilities are available:

1. add a documented data-heavy/widget-integration recipe or reference example;
2. show typed Mendix props/datasource handling with Mendraw;
3. show props-driven keyed remounting and async external-module initialization with Glendix;
4. show browser download/file selection without local Blob/input JavaScript;
5. update generated-project guidance so local FFI remains a last-resort, typed platform boundary;
6. keep the existing simple template lean rather than adding all dependencies to every generated project.

Acceptance

• Reference source and docs follow the existing “widget logic/UI in Gleam” rule.
• Generated simple projects do not gain unnecessary dependencies or boilerplate.
• Any new optional recipe builds and tests successfully.
• Downstream widget-build and family E2E paths pass when the change affects widget output.