clearForNid method
Defensive slot reset for _adoptKey and _releaseNid. Bounds-checks
in case the engine's array hasn't grown to nid yet (rare but
possible during initialization races).
Implementation
void clearForNid(int nid) {
if (nid < 0 || nid >= _slideByNid.length) return;
final prev = _slideByNid[nid];
if (prev != null) {
if (prev.startDeltaX != 0.0) _xActiveCount--;
_slideByNid[nid] = null;
_activeSlideNids.remove(nid);
}
}