certain actions need to happen when
- starting restore()
- switching date within restore()
- stopping restore()
pretty good fit for an object and raii. so, using context struct to handle
sun{rise,set} initialization and per-schedule time match updates
sun{rise,set} event happens on a specific date, make sure it is also checked
fix double action trigger by comparing minutes, not raw timestamp seconds
experimenting with 'invalid state' i.e. when timestamp was not generated
include restore logic in tests, as code now lives in .ipp
