[ZhuchkaTriplesix]

Closes #1520.

#1614 made the comma copy conditional; this removes it. Selection stats (Sum / Avg / Min / Max under the grid) parsed every cell with a comma as double.tryParse(cell.replaceAll(',', '')), so a drag over formatted numbers made one string per cell on every update.

Change

• GridSelectionCalcEngine.parseThousandsSeparated reads the number from the code units:
  • commas are skipped wherever they are, exactly like the copy they replace (so 1,5 is still 15, as before; changing that is a separate question);
  • with up to 15 significant digits and a power of ten up to 22 the value is computed exactly: an integer below 2^53 times or divided by an exactly representable power of ten is a single correctly rounded operation;
  • longer numbers fall back to the copy + double.tryParse.
• Cells without a comma still go straight to double.tryParse, which needs no copy.
• trim() already returns the cell itself when there is nothing to trim, so a plain cell allocates no strings.

Checked

A standalone Dart script compared it with double.tryParse(s.replaceAll(',', '')) (results, sign of zero, NaN / infinity treated as non-numbers like the engine does):

• 300,000 random strings over 0-9 , . - + e E;
• 200,000 money-shaped values (1,234,567.89);
• edge cases: 5., .5, 1e22 / 1e23, 19+ digits, 9,007,199,254,740,993, NaN, Infinity.

No difference. Speed on 5,000 comma cells: 172 µs per pass instead of 382 µs, with no allocations.

Tests

grid_selection_calc_format_test.dart compares it with the copy on a fixed list (signs, fractions, exponents, commas in odd places, long numbers, exponent overflow), checks -0, and checks that text with commas is not a number.

Not run with flutter test locally; CI decides.