Fix compilation error in KdtreeBoundingBoxNearestNeighbors

[kongyuanchao]

Summary

This PR fixes the swap function implementation in KdtreeBoundingBoxNearestNeighbors by updating the swapped member variables to match the actual class members.

Changes
Before (incorrect swap members):
swap(this->mean, other.mean);
swap(this->metric, other.metric);
swap(this->samples, other.samples);
swap(this->root, other.root);
swap(this->values, other.values);
swap(this->var, other.var);

After (corrected to existing class members):
swap(this->boundingBox, other.boundingBox);
swap(this->metric, other.metric);
swap(this->checks, other.checks);
swap(this->root, other.root);
swap(this->values, other.values);
swap(this->nodeDataMax, other.nodeDataMax);

Reason
The previous implementation attempted to swap non-existent or outdated members (mean, samples, var), leading to build errors such as:

error: 'class rl::math::KdtreeBoundingBoxNearestNeighbors' has no member named 'var'
This PR corrects that by aligning the swapped fields with the actual member variables of the class.

Environment Tested
Windows 11 / Git Bash
GCC via MSYS2 / MinGW64

Manual build successful after fix