resizeForCapacity method
Grows the per-nid slide array to match nidCapacity. Called by
TreeController._onStoreCapacityGrew in lockstep with every other
per-nid array. New slots default to null (not sliding).
Implementation
void resizeForCapacity(int nidCapacity) {
if (nidCapacity <= _slideByNid.length) return;
final grown = List<SlideAnimation<TKey>?>.filled(nidCapacity, null);
for (int i = 0; i < _slideByNid.length; i++) {
grown[i] = _slideByNid[i];
}
_slideByNid = grown;
}