perf: Chart control performance improvements

[PaulAndersonS]

Root Cause of the Issue

The Chart control source code uses several LINQ patterns (.Reverse(), .Cast<>(), .Where().ToList()) and repeated string operations that cause unnecessary heap allocations and repeated work on every frame/update cycle.

Description of Change

This PR implements 10 targeted performance improvements in the Chart control source code:

1. Replace .Reverse() with reverse index loop in ChartBase.cs — avoids allocating a reversed collection during tooltip hit-testing
2. Remove .Where().ToList() allocation in CartesianSeries.cs — replaces filtered list with direct iteration
3. Replace .Cast<CartesianSeries>() with pattern matching in ChartAxis.cs — eliminates LINQ enumerator overhead
4. Cache IndexOf calls in ChartSeriesPartial.cs — avoids scanning the same string multiple times
5. Replace string concatenation with interpolation in ChartSeries.cs — minor allocation improvement
6. Replace .Cast<CartesianSeries>() in group update in ChartSeriesPartial.cs — pattern matching loop
7. Simplify ItemsSource IList check — already optimized in current code (no change needed)
8. Cache .Count in loop conditions in CartesianAxisRenderer.cs — avoids repeated property access
9. Cache complex property path check in ChartSeriesPartial.cs — introduces _isComplexXProperty field to avoid repeated String.Contains calls
10. Replace remaining .Cast<>() usages in CartesianSeries.cs legend item loops — pattern matching with indexed access

Issues Fixed

N/A — Performance improvement

Screenshots

N/A — No visual changes