clearForNid method
Per-nid cache cleanup used by the controller's adopt/release paths.
Zeros both the subtree-size slot and the reverse-index slot in one
call. Idempotent — safe to call on already-cleared slots. nid must
be in range (callers that only have a key should resolve nid via the
registry first).
Implementation
void clearForNid(int nid) {
if (nid >= 0 && nid < _subtreeSizeByNid.length) {
_subtreeSizeByNid[nid] = 0;
}
if (nid >= 0 && nid < _indexByNid.length) {
_indexByNid[nid] = kNotVisible;
}
}