Remove step 1 cache after saving the session in command-line mode

[michalpelka]

Fixes #489

Problem

The step 1 cache directory (<working_directory>/cache) was only removed by the GUI's on_exit (window close). The headless modes of lidar_odometry_step_1 — <input_folder> and <input_folder> <params.toml> <output_folder> — never removed it, leaving all cached chunk points on disk after every run.

Change

• New remove_cache(params) helper in lidar_odometry.cpp (skips an empty path, reports a failure via std::error_code instead of throwing).
• Called after the session is saved in both command-line branches of lidar_odometry_gui.cpp.
• on_exit in lidar_odometry_gui.cpp and the precision_forestry app now uses the same helper.

In GUI mode the cache is still removed on window close, not right after the auto-save: "Save result", "Point cloud consistency…" and "Save all point clouds to single las/laz" still read the cache files after that point.

Testing

• lidar_odometry_step_1 and drag_folder_with_mandeye_data_and_drop_here-precision_forestry build (macOS, release).
• Not yet run on real data.

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