DxfImport_ArcBoundingBoxes_Diagnostic read a drawing from one user's
desktop, returned early elsewhere and asserted nothing, so the near-zero
sweep bounding-box fix (4053f1f) had no regression test that could fail.
Replace it with ArcBoundingBoxTests: a forward and a reversed arc with a
sweep below Tolerance.Epsilon must be bounded by its endpoints, and an
ordinary arc crossing 90 degrees must still reach the top of its circle.
With the 4053f1f guard reverted, both near-zero cases fail (the box grows
to the full circle); with it in place all three pass.
aj
a854e34a3d