FDTD | Feature | Add VTK observation output

[adrianarce-elemwave]

Summary

Adds the initial VTK XML output implementation for observation geometry.

Changes

• Adds vtkAPI_m for ASCII .vts and .vtu output, including points, data arrays, and unstructured-cell connectivity.
• Adds mapVTKOutput_m to collect material-boundary geometry, exclude PML regions, create line/quad topology, and write .vtu files.
• Generates a companion metadata file describing material-tag values.

Stack Note

This PR supplies the VTK output implementation layer. A follow-up integration PR must add the modules to CMake and invoke them from the output workflow.