App-wide template defaults for the [cngxMatTabs] decorations -
the middle cascade tier between a per-instance structural slot and
the built-in fallback. Sibling of CngxTabsTemplates /
CngxStepperTemplates so consumer templates port across families.
Default for the rejection SR-descriptor content
(*cngxMatTabRejectionContent). Per-instance slot still wins;
without either tier the imperative fallback writes the resolved
i18n phrase verbatim.