perf(grid): allocation-light selection export (#1524)

[ZhuchkaTriplesix]

Summary

• toCsv / toTsv created a List<String> per row and joined it. Cells are now written straight into the StringBuffer, with the delimiter before every column except the first. The final trimRight() stays, so the output is byte for byte the previous one (including the quirk that trailing empty cells of the last TSV row are trimmed).
• toJson compiled RegExp(r'^0\d+$') for every cell. It is _hasLeadingZeroDigits now (a 0 followed by one or more ASCII digits, on the trimmed text), and int.tryParse / double.tryParse results are used directly instead of parsing twice.

Not done: the per-row Map in toJson. JsonEncoder needs the maps, and hand-writing JSON for the clipboard is a larger change than this issue's regex and list costs.

Tests (not run locally; CI will)

test/features/workspace/grid_selection_export_test.dart: CSV and TSV with and without header, quoting of commas / quotes, a column sub-range with an out-of-range column and row, the TSV trailing-cell trim, empty input, and JSON for leading zeros, padded zeros, 0, 0.5, NULL, booleans, plus a multi-row array. Existing context-menu tests are untouched.

Closes #1524