Include form-associated custom elements in form submissions - #807

This pull request fixes the original issue #593.

I develop Viewflow.io and am evaluating alternatives to Turbolinks, Hotwire/Turbo, and htmx. During that work, I found a discrepancy in Unpoly. An [up-submit] form dropped values from form-associated custom elements. A normal browser submission included them.

What changes

up.Params.fromForm() now begins with native FormData(form). It uses the browser's form-data construction algorithm. This includes form-associated custom elements and values appended by formdata listeners.

The method then adds configured Unpoly fields that are absent from form.elements. This keeps up.form.config.fieldSelectors working for non-native controls. It does not duplicate native or form-associated controls.

This is a small submission-only change. It does not change up.Params.fromContainer() or Unpoly's watcher behavior.

Compatibility

Existing native controls keep browser submission semantics. Configured custom controls still submit when they expose name, value, and disabled properties.

Verification

• I added specs for formdata listeners, form-associated custom elements, and configured non-native controls.
• I ran npm run lint, npm run build-ci, and the focused Chrome up.Params suite.

Authorship

This PR was AI-assisted and made with Codex Terra. I reviewed every line and take responsibility for the code.