feat(io): add unified save_dataset entry point (#965)

[AshNicolus]

Description

Closes #965

Adds save_dataset, a single entry point for saving movement datasets that
mirrors load_dataset on the write side. Previously users had to reach for the
format-specific to_*_file writers (or raw xarray) directly.

from movement.io import save_dataset

save_dataset(ds, "out.nc")                                   # native netCDF (default)
save_dataset(ds, "out.h5", source_software="DeepLabCut")     # poses → DLC
save_dataset(ds, "out.csv", source_software="VIA-tracks")    # bboxes → VIA

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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