perf(grid): eliminate per-row List allocations and dynamic regex in clipboard export #1524

Problem

In GridSelectionManager / clipboard formatting logic (lib/features/workspace/grid/grid_selection_manager.dart), exporting table data creates heavy GC overhead:

1. toCsv and toTsv allocate a new final cells = <String>[]; on every single row, followed by cells.join(...). For 10,000 rows, this creates 10,000 intermediate lists.
2. toJson instantiates RegExp(r'^0\d+$') dynamically on cell evaluations and builds Map<String, dynamic> for each row before serialization.

Solution

• Stream cell contents directly into StringBuffer using index checks: if (c > startCol) buffer.write(delimiter).
• Replace dynamic RegExp with static cached regex or integer code unit checks (codeUnitAt).
• Eliminate intermediate row lists and per-row maps.

Benefits

• 0 intermediate list allocations during CSV/TSV copy.
• Significant throughput boost when copying tens of thousands of cells.