Application-wide default configuration for every CngxTreetable
instance in the injection scope.
Precedence cascade. Each treetable instance reads its effective
options through CngxTreetable.resolvedOptions which overlays
the per-instance [options] input on top of this app-wide config.
Per-instance wins for the keys it sets; app-wide fills the rest;
library defaults fill anything still unset. Same shape as Material's
MAT_* defaults pattern.
Register an instance of this via provideTreetable; reach it
by injecting CNGX_TREETABLE_CONFIG in custom code (rare -
CngxTreetable already does that internally).