feat: VTK/ParaView export for interior field visualisation

[timini]

Summary

Export the FEM solution field to VTK format for visualisation in ParaView, enabling interior pressure field inspection at each frequency.

Motivation

Currently the pipeline only outputs SPL at the outlet surface as a scalar. Being able to visualise the full 3D pressure field inside the horn is essential for:

• Understanding mode shapes and standing wave patterns
• Debugging unexpected SPL behaviour
• Producing publication-quality field plots

Implementation Notes

• dolfinx has built-in VTK/XDMF export via dolfinx.io.VTXWriter or dolfinx.io.XDMFFile
• Export the complex pressure magnitude |p(x)| at each frequency step
• Consider XDMF format for time-series (frequency-series) data

Priority

P1 — Near-term