LAS export: point_source_id not set — cloud identity lost in multi-scan exports #427

When exporting a session with multiple point clouds to a .las / .laz file via save_all_marked_scans/global scan(), all points receive point_source_id = 0. This makes it impossible to distinguish which scan each point came from in downstream tools (CloudCompare, PDAL, LAStools, etc.) that rely on point_source_id as the scan number.