perf: Reduce allocations and improve algorithmic efficiency across Chart controls

[PaulAndersonS]

Root Cause of the Issue

Multiple performance anti-patterns exist in Chart control rendering and layout paths that cause unnecessary memory allocations and CPU usage:

• Array allocation just to access a single element
• String concatenation in loops creating O(n) intermediate string objects
• Lists created without capacity hints in rendering hot paths causing repeated resizing
• Multiple string concatenation operators creating intermediate string allocations

Description of Change

Five targeted performance optimizations:

1. AxisLabelLayout.cs — Replace Keys.ToArray()[lastIndex] with Keys.ElementAt(lastIndex) to avoid allocating an entire array just to access the last key during axis label intersection checks.

2. CategoryAxis.cs — Replace string + ", " + label concatenation in a loop with StringBuilder in GetLabelContent(). This reduces string allocations from O(n) to O(1) when generating grouped category labels.

3. PolarAreaSegment.cs — Pre-allocate List<float> with known capacity (_pointsCount + 2) * 2 in both GenerateInteriorPoints() and GenerateStrokePoints(). These methods run on every frame during animation, so avoiding List resizing directly reduces GC pressure.

4. BoxAndWhiskerSeries.cs — Replace 5-way string concatenation with string interpolation in tooltip text generation to reduce intermediate string allocations.

5. HiLoOpenCloseSeries.cs — Replace 4-way string concatenation with string interpolation in tooltip text generation.

Issues Fixed

N/A — Proactive performance improvement

Screenshots

N/A — Internal optimization with no visual changes