perf(grid): fast zero-allocation numeric stat formatting in GridSelectionCalcEngine #1531

Problem

In GridSelectionCalcEngine.formatNum (lib/features/workspace/grid_selection_calc_engine.dart), whenever the user drags or modifies a selection in the data grid:
formatted.replaceAll(RegExp(r'0+$'), '').replaceAll(RegExp(r'\.$'), '')
is invoked for each summary metric (Sum, Avg, Min, Max).
This dynamically compiles two RegExp instances and runs two regex replacement passes on every pointer movement.

Solution

• Trim trailing 0 and . characters using a zero-allocation backward index scan on code units (codeUnitAt).
• Only perform a single substring slice when trailing characters actually exist.

Benefits

• 0 RegExp compilations during continuous cell selection drag.
• Zero GC churn in status bar summary updates.