Buffer.fromCells constructor
Creates a buffer from pre-built cells without applying Line.set().
Upstream tests construct expected buffers directly (without triggering overwrite logic), so this helper lets Dart parity tests do the same.
Implementation
factory Buffer.fromCells(List<List<Cell>> cellLines) {
final lines = cellLines.map(Line.fromCells).toList(growable: false);
return Buffer._(lines);
}