Perf: Hoist ToList() out of loops in CartesianChartArea.UpdateSBS and GetTotalWidth

[PaulAndersonS]

Root Cause of the Issue

In CartesianChartArea.cs, both UpdateSBS() and GetTotalWidth() called SideBySideSeriesPosition.Values.ToList()[i] inside a for-loop. This creates a brand new List<List<CartesianSeries>> on every iteration just to index into it — resulting in O(n) unnecessary heap allocations where only 1 is needed.

For charts with many series groups, this wastes memory and puts pressure on the garbage collector during layout calculations.

Description of Change

Hoisted the .Values.ToList() call before the loop in both methods, materializing the dictionary values once and reusing the cached list for indexed access.

Before (called N times):

for (int i = 0; i < SideBySideSeriesPosition.Count; i++)
{
    var seriesGroup = SideBySideSeriesPosition.Values.ToList()[i]; // allocates every iteration
}

After (called once):

var seriesGroups = SideBySideSeriesPosition.Values.ToList(); // single allocation
for (int i = 0; i < seriesGroups.Count; i++)
{
    var seriesGroup = seriesGroups[i];
}

Also added a unit test UpdateSBS_WithMultipleSeriesGroups_ShouldComputeSbsInfo to verify correct behavior with multiple series groups.

Issues Fixed

Performance improvement — no specific issue.

Screenshots

N/A — internal performance optimization with no visual change.