anywidget loader rejects legacy named `render` export; error message is unhelpful

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Summary

marimo's anywidget loader only recognises a widget ESM that has a default
export. An ESM that exposes its lifecycle via the legacy named export —

export function render({ model, el }) { ... }

is rejected with a generic, hard-to-diagnose error:

Module at ./@file/<hash>.js does not appear to be a valid anywidget

anywidget's own runtime still loads this named-export form (it's deprecated, so
it emits a console deprecation warning, but it renders). The same widget that
works in Jupyter / JupyterLab therefore fails to render in marimo.

Two separate asks:

1. Error message — make it actionable.
2. Compat — optionally accept the deprecated named render/initialize
   export for parity with anywidget's runtime.

Environment

• marimo 0.23.10
• anywidget 0.11.0

Steps to reproduce

Any anywidget whose _esm uses the legacy named export, e.g.:

import anywidget, traitlets

class W(anywidget.AnyWidget):
    _esm = """
    export function render({ model, el }) {
        el.textContent = "hello from named render";
    }
    """

W()

• Colab: colab link
• marimo: molab link

Requested changes

1. Improve the error message

The current message gives the user no path forward. Inspect the imported module
and say why it failed and what to do — for example:

The module at <url> is not a valid anywidget front-end module: it has no
default export. Export your widget as export default { render } (see
https://anywidget.dev/en/afm/). Note: a bare export function render is the
deprecated anywidget API and is not supported here.

Even just detecting "module has a named render/initialize but no default
export" and emitting a targeted hint would save a lot of debugging.

2. (Optional) Accept the deprecated named export

To match anywidget's own loader, Px could also accept top-level
render/initialize named exports as a fallback, e.g.:

function isValidAnywidget(t) {
  const d = t.default;
  if (d) {
    return typeof d === "function"
      || typeof d?.render === "function"
      || typeof d?.initialize === "function";
  }
  // Backward-compat with anywidget's deprecated named-export API.
  return typeof t.render === "function" || typeof t.initialize === "function";
}

Related to fsspec/projspec#103