DI token a CngxButtonToggle leaf injects to find its single-mode
parent group. Always provided via
{ provide: CNGX_BUTTON_TOGGLE_GROUP, useExisting: CngxButtonToggleGroup } - never via concrete-class injection (per
reference_atomic_decompose ยง4).
The leaf injects this token AND CNGX_BUTTON_MULTI_TOGGLE_GROUP
with { optional: true } on both; the present token determines the
leaf's keyboard semantics + ARIA pattern (aria-checked for single,
aria-selected for multi), chosen at injection time, NOT at
runtime - there is no [selectionMode] flag.