[ZhuchkaTriplesix]

Summary

• perf(grid): fast zero-allocation numeric stat formatting in GridSelectionCalcEngine #1531. formatNum compiled RegExp(r'0+$') and RegExp(r'\.$') and ran two replace passes for each of Sum / Avg / Median / Min / Max / StdDev on every selection change. It now walks the code units from the end: drop trailing 0s, then one ., and slice once only if something was removed. Integer-valued numbers still go through the toInt() branch.
• perf(grid): eliminate string allocations in GridSelectionCalcEngine via zero-allocation formatted number scanner #1520 (part). compute made trimmed.replaceAll(',', '') for every selected cell. It now makes the copy only for cells that contain a comma; the value that goes to the distinct set and to tryParse is unchanged.

Not done: the custom allocation-free number parser the #1520 issue proposes. double.tryParse is correctly rounded and a hand-written parser has to match it digit for digit, which is a bigger risk than the copy it saves, so that issue stays open (Refs).

Tests (not run locally; CI will)

grid_selection_calc_format_test.dart: formatNum on whole, fractional, repeating and round-to-integer values, and compute on a mix of 1,000, padded 2,500.5, NULL, empty, text and duplicate values (counts, distinct, sum, min, max, median).

Closes #1531
Refs #1520