…ce (#3232)

The widget resolved its fullscreen affordance through a four-way `??` chain:
a `mobileFullscreen` (camelCase) prop, `field.mobile_fullscreen`, a
`mobile_fullscreen` prop, and `schema.mobile_fullscreen`. Three of the four
were permanently `undefined`:

- `mobileFullscreen` had no producer anywhere in the repo — the only two
  occurrences of that spelling were this widget's own read and the destructure
  that kept it off the DOM spread. The doc comment claimed "the host form
  passes `mobileFullscreen`", so it documented a contract that never held.
- `mobile_fullscreen` as a prop cannot arrive: `stripRegisteredFieldProps`
  (components/src/renderers/form/form.tsx) removes it, and `fullscreen`, from
  the props forwarded to registered field widgets.
- `schema.mobile_fullscreen` restates the object `field || schema` already
  resolves into `textareaField`.

What is left is the one live source: the field metadata flag ObjectForm stamps
onto long-text fields from `ObjectFormSchema.mobile.fullscreenLongText`. The
doc comment now describes that, names the producer, and records why there is
deliberately no host-override prop.

No behaviour change. The metadata path is pinned by new tests covering the
button, the dialog and the committed edit, so the cleanup cannot have removed
the working affordance; the retired spellings are pinned inert at compile time
(they are not on the closed `FieldWidgetComponentProps`, #3221) and at runtime.

The built-in (unregistered) `textarea` branch of the form renderer, which reads
`mobile_fullscreen || fullscreen` off the form-field props and renders its own
`FullscreenTextarea`, is a separate live path and is untouched.

Per AGENTS.md #0.1 and Prime Directive #12: divergence converges at the
producer, not by accumulating tolerance at the consumer.

Fixes #3232

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NVPjPzmmAJ2Ngtvgg5MSRa