feat: export for CAD, ParaView and OpenFOAM

[PSamuele]

What

New module naca_core/export.py. At the end of a run both optimizers write an export/ subfolder with the optimized airfoil in four formats, scaled with the chord entered in the setup:

File	For
<name>.dxf	CAD: closed 2D profile in mm (spline through the points + trailing-edge line), ready to extrude
<name>_mm.csv	CAD: x,y,z points in mm, no header, trailing edge left open
<name>_surface.vtk	ParaView: airfoil contour in m with Cp and V/Vinf from the in-house panel method
<name>_wing.stl	OpenFOAM (snappyHexMesh), CAD, 3D printing: airfoil extruded along z, watertight, in m

Other changes:

• New prompt Enter span for the 3D export (m), default = chord. It only sets the length of the extrusion in the STL.
• ezdxf added to requirements.txt and to the start-up library check.
• README: new section "Using the exported files" (CAD, ParaView, OpenFOAM), export files in the Output table, and the export values in the Reference table.

The .dat file is unchanged: it is the format XFOIL needs.

Verified

• Five airfoils, including the extremes of the search range: DXF in mm and closed (the spline stays within 7 micrometres of the exact NACA curve for a 250 mm chord); STL watertight with consistent normals and volume equal to area x span; VTK read back with the VTK library (the ParaView engine), Cp identical to the panel method; CSV points and scale correct.
• OpenFOAM v1912: surfaceCheck reports a closed surface, one zone, consistent normals, no illegal triangles; snappyHexMesh removes the cells inside the airfoil and ends with "Finished meshing without any errors".
• Windows: full in-house run with chord 0.25 m. The CSV spans 0 to 250 mm in x and 28 mm in height (t = 0.1118 x 250 = 27.95 mm), 399 points. The DXF opens in CAD as a closed profile at the right size.

Depends on

#14 (base branch: chore/remove-example-results).