fix: add setter for rendererXAxis to match rendererLeftYAxis/rendererRightYAxis API

[mvanhorn]

Summary

• Converts rendererXAxis from a read-only val (getter only) to a var with both getter and setter in BarLineChartBase
• The setter writes to the protected xAxisRenderer backing field, following the identical pattern already used by rendererLeftYAxis and rendererRightYAxis
• Adds KDoc comment on the property consistent with the Y-axis renderer docs

Why this matters

Issue #782 reports that callers cannot substitute a custom XAxisRenderer subclass the same way they can for Y-axis renderers. The asymmetry is a gap in the API: rendererLeftYAxis and rendererRightYAxis are both var with setters, while rendererXAxis was val. This is a common customization (e.g., custom label formatting or line drawing). The backing field xAxisRenderer is already declared protected var, so this change requires no visibility changes - only the property accessor needed updating.

Testing

• Verified the new setter mirrors the rendererLeftYAxis setter pattern exactly (backing field assignment)
• Kotlin syntax inspection confirms no errors - xAxisRenderer is a protected var accepting XAxisRenderer assignment
• Full Android Gradle build was not run locally (requires Android SDK); the change is a single property accessor addition with no logic

Fixes #782