Add VTU importer with surface + volume cell partitioning

[tarcila]

This improves on the VTU importer so:

• it is providing SpatialField for the -volume command line importer
• it is providing both SpatialField and surfaces when using the new -vtu

[Copilot]

Pull request overview

This pull request enhances the VTU importer to support both surface and volume cell partitioning, providing a more comprehensive import capability for VTK Unstructured Grid files. The changes enable the -vtu command-line option to import both surfaces (triangles, quads, polygons) and volumes (tetrahedra, voxels, hexahedra, wedges, pyramids) from a single VTU file, while maintaining backward compatibility with the -volume option that only imports volume cells as a SpatialField.

Changes:

• Added new VTU importer with dual functionality: full scene import with surfaces + volumes, and spatial field-only import for backward compatibility
• Implemented cell type partitioning to separate surface cells (for mesh rendering) from volume cells (for spatial field rendering)
• Integrated the new importer into the command-line interface and file import dispatcher

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
tsd/src/tsd/io/importers/import_VTU.cpp	Complete rewrite with surface extraction/triangulation and volume cell filtering logic
tsd/src/tsd/io/importers.hpp	Added declaration for new void import_VTU(Scene&, const char*, LayerNodeRef) overload
tsd/src/tsd/io/importers/import_file.cpp	Integrated VTU importer into the file dispatch logic
tsd/src/tsd/app/Core.cpp	Added -vtu command-line flag handling

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[jeffamstutz]

LGTM, thanks!