Add PointsLoad::fromMultiScanFolder to load and merge a multi-scan laser folder

[Fedr]

Summary

Adds a new function to MRMesh for loading the output of a multi-scan laser capture (a folder with many individual scans) and merging it into a single point cloud:

namespace MR::PointsLoad
{
    MRMESH_API Expected<PointCloud> fromMultiScanFolder(
        const std::filesystem::path& folder, const ProgressCallback& callback = {} );
}

What it does

1. Takes a folder path.
2. Returns Expected<PointCloud>.
3. Finds in the folder all pairs of _intempNNN.pose and _laserNNN.ply files, matched by the numeric index NNN (the number of pairs is arbitrary).
4. Reads each .pose file as an AffineXf3f (a 4×4 row-major rigid transformation) and applies it to the points (and rotates the normals) loaded from the .ply file with the same index.
5. Returns the union of all transformed points, expressed in the common coordinate frame.

Indices missing a counterpart (.pose without .ply or vice versa) are skipped; if no pairs are found at all an error is returned. Progress is reported per processed pair and the operation is cancelable.

The folder layout matches the export of a laser scanner (e.g. Revopoint), where _intempNNN.pose holds the per-scan pose and _laserNNN.ply holds the corresponding scanned points with normals.