Invert bulge value calculation in import_dxf.py

[greyltc]

Invert the bulge value for the start point in the import_dxf function. Fixes #1368

[Copilot]

Pull request overview

This PR adjusts DXF polyline bulge handling so imported bulged segments (arcs) curve in the expected direction, addressing the “inverted arcs” behavior reported in #1368.

Changes:

• Invert the per-segment bulge sign used when converting DXF polylines into SagittaArc edges.

Comments suppressed due to low confidence (1)

src/build123d/import_dxf.py:216

• math.dist(start_point, end_point) is computed twice per segment (degeneracy check and sagitta calculation). Cache the chord length once to avoid redundant sqrt work and keep the loop easier to read; you can also simplify the sign inversion to -start_data[2].

        bulge = -1 * start_data[2] if len(start_data) > 2 else 0

        if math.dist(start_point, end_point) < TOLERANCE:
            continue

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