feat(core): add cell address parsing and column labels

lib/src/address.dart

@@ -0,0 +1,64 @@
+final RegExp _addressPattern = RegExp(r'^\$?([A-Za-z]+)\$?([0-9]+)$');
+
+int columnIndex(String letters) {
+  var index = 0;
+  for (final unit in letters.toUpperCase().codeUnits) {
+    index = index * 26 + (unit - 0x41 + 1);
+  }
+  return index - 1;
+}
+
+String columnLabel(int index) {
+  var remaining = index + 1;
+  final units = <int>[];
+  while (remaining > 0) {
+    final position = (remaining - 1) % 26;
+    units.add(0x41 + position);
+    remaining = (remaining - 1) ~/ 26;
+  }
+  return String.fromCharCodes(units.reversed);
+}
+
+class CellAddress {
+  final int col;
+  final int row;
+
+  const CellAddress(this.col, this.row);
+
+  factory CellAddress.parse(String reference) {
+    final match = _addressPattern.firstMatch(reference.trim());
+    if (match == null) {
+      throw FormatException('Invalid cell reference: $reference');
+    }
+    return CellAddress(columnIndex(match.group(1)!), int.parse(match.group(2)!) - 1);
+  }
+
+  String get label => '${columnLabel(col)}${row + 1}';
+
+  @override
+  bool operator ==(Object other) => other is CellAddress && other.col == col && other.row == row;
+
+  @override
+  int get hashCode => Object.hash(col, row);
+
+  @override
+  String toString() => label;
+}
+
+List<CellAddress> expandRange(CellAddress start, CellAddress end) {
+  final minCol = start.col < end.col ? start.col : end.col;
+  final maxCol = start.col > end.col ? start.col : end.col;
+  final minRow = start.row < end.row ? start.row : end.row;
+  final maxRow = start.row > end.row ? start.row : end.row;
+  final cells = <CellAddress>[];
+  for (var row = minRow; row <= maxRow; row++) {
+    for (var col = minCol; col <= maxCol; col++) {
+      cells.add(CellAddress(col, row));
+    }
+  }
+  return cells;
+}
+
+int rangeRowCount(CellAddress start, CellAddress end) => (start.row - end.row).abs() + 1;
+
+int rangeColCount(CellAddress start, CellAddress end) => (start.col - end.col).abs() + 1;