Support dynamic labels (<output>) in groups

### Description

Currently, Web Forms does not support dynamic group labels. A group `` element containing `` expressions that reference fields within the group is not evaluated reactively.

Example form: [dynamic_label.xml](https://github.com/user-attachments/files/30046437/dynamic_label.xml)

XForms spec: The spec defines `` as a reactive element where its value expression should be re-evaluated whenever its dependencies change, regardless of context (group or repeat).

Behavior across renderers:
- Web Forms: No support — label is not rendered dynamically
- Enketo: No support
- ODK Collect: Partial — label updates only when navigating away from and back to the group (page re-render), not while typing

Dynamic `` in repeat labels is already supported in Web Forms. The question is whether we want to extend that support to plain (non-repeat) groups.

### User Stories

- As a data collector filling out a form,
I want group labels that include expressions to update as I enter data,
so that I can see contextual information (e.g., a respondent's name) reflected in the group header without having to navigate away and back.

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### Additional Notes

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### Related Issues

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Start with dynamic_label.xml and the existing Web Forms behavior for dynamic in repeat labels. Compare how dependency changes update repeat labels with how plain group labels render. Done means group labels re-evaluate while referenced fields are edited, matching the stated XForms behavior.

Written by the indexing model from the issue text.