setCell method
Paints a single character at (x,y); out-of-bounds writes are ignored.
A multi-rune char is truncated to its first rune; an empty string paints
a space.
Implementation
void setCell(int x, int y, String char, Style style) {
if (x < 0 || x >= width || y < 0 || y >= height) return;
final runes = char.runes;
final c = runes.isEmpty ? ' ' : String.fromCharCode(runes.first);
_cells[_index(x, y)] = Cell(c, style);
}