Design docs

Design docs record why a non-trivial change was made — the alternatives weighed and why one won. They are dated snapshots of a decision, not living documentation; where one disagrees with the rest of this site, the site is current and the doc is history.

• Section geometry on an arbitrary plane — generalising the horizontal-only footprint cut to any plane (issue #3, shipped in v0.2.0).
• Outward-facing classification — deciding which way a building element faces, and which of its two sides is the exposed one (issue #4).