insert method
Inserts item at position (x, y). The same item may be inserted at
multiple positions; each insertion is independent.
Example:
final SpatialGrid<String> g = SpatialGrid<String>(10)
..insert('a', 1, 1)
..insert('b', 5, 5);
g.queryRadius(0, 0, 3); // ['a']
Audited: 2026-06-12 11:26 EDT
Implementation
void insert(T item, double x, double y) {
final (int, int) key = (_cell(x), _cell(y));
_cells.putIfAbsent(key, () => <_Entry<T>>[]).add(_Entry<T>(item, x, y));
_size++;
}