feat: flexible boundary tagging (replace z-coordinate heuristic)

[timini]

Summary

Replace the current z-coordinate centre-of-mass heuristic for boundary tagging with a more robust approach that supports non-axial geometries.

Motivation

The current boundary tagging identifies inlet/outlet/wall surfaces by checking if their centre of mass is at z=0 or z=length. This breaks for:

• Folded horns (inlet and outlet may be at the same z-coordinate)
• Horns with non-planar mouths
• User-supplied STEP files with arbitrary orientation

Implementation Notes

• Option A: Use named physical groups in the STEP/BREP file (set in CAD)
• Option B: Surface normal analysis (inlet/outlet have normals along the horn axis)
• Option C: Interactive or config-file-based surface selection
• gmsh supports physical group names that could be standardised (e.g., "inlet", "outlet", "wall")

Priority

P2 — Medium-term