An owned value to interpolate into a localized message. Owned (rather than a
borrowed FluentValue) so callers — most importantly the retained-mode
Text widget — can stash arguments alongside a translation key and re-format
every frame without keeping the original references alive.
Layout-stress translation mode used by screenshots and acceptance tests.
It deliberately preserves digits and punctuation while expanding letters,
making clipped labels and fixed-width controls visible before translators
have completed a catalog.