Design and ADRs

• DESIGN.md: domain semantics and an example for every edge case.
• Architecture decision records
  • 0000. Record architecture decisions
  • 0001. Minimum supported Rust version
  • 0002. Pin GitHub Actions by commit SHA
  • 0003. Python package layout
  • 0004. jiff for dates, times and time zones
  • 0005. abi3 wheels for Python
  • 0006. WebAssembly via wasm-bindgen over napi-rs
  • 0007. businesstimedelta compatibility strategy
  • 0008. Coverage in GitHub Actions
  • 0009. npm package layout and wasm-pack targets