Set point_source_ID per scan when exporting LAZ/LAS

[piorkoo]

Summary

• Each exported point now carries point_source_ID equal to the source scan's index in point_clouds_container.point_clouds, so merged exports remain attributable to their source scan.
• Covered export paths: Save all marked scans → Local scan / Global scan (save_all_to_las), Separate global scans (laz/las) (save_separately_to_las → save_processed_pc), and Save all marked trajectories (save_trajectories_to_laz → exportLaz).
• Capped at 65535 (limit of unsigned short) with a stderr warning when the session contains more scans. Indexing follows position in the full point_clouds vector — hidden scans keep their original ID.

Test plan

• Rebuild multi_view_tls_registration (CMake).
• Load a session with ≥ 3 scans, export Global scan to .laz; verify in CloudCompare or lasinfo -histo point_source that point_source_ID values match scan indices.
• Repeat with Local scan (IDs unchanged by transform).
• Run Separate global scans (laz) — each output file should have a single uniform point_source_ID equal to its scan index.
• Run Save all marked trajectories → Save all as las/laz files — trajectory points inherit psid per scan.
• Verify backward compatibility: existing exports still open (point format 1, record length 28 unchanged).

🤖 Generated with Claude Code

[michalpelka]

I like code, lets run CI.