kernel::sweep swept segment by segment: a line translates the profile along it, an arc turns it. At a sharp corner the next line runs within the plane the profile has reached, so the profile slid sideways in its own plane and the walls folded into an inside-out, zero-volume shell. Paths whose direction changes at a joint (more than 1e-3 rad) are now swept as a polyline by sweep_path, which mitres the corners; arcs become points within 0.01 mm. Smooth paths (lines and tangent arcs) are swept as before. Pipe uses the same path.
Tests: sweep_round_a_sharp_corner_is_mitred sweeps a 2 x 2 square along an L of two 10 mm lines and checks the volume, 80 mm3 (fails on main).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
