v0.2.0

Added

• Modelled ISO metric threads that run out into chamfers and countersinks.

Changed

• Fillets at corners and hole rims are built together, with exact end arcs and
  finer round fillets.
• Shells follow fillets.
• Exact lines and circles on boolean edges, and smooth STEP intersection curves.

Fixed

• Inside corners where three edges meet are rounded.
• Fillets that leave the solid unchanged fail instead of passing silently.
• Edge gaps are checked in STEP output.