How a treatment row is scheduled. This, not `frequency`, decides whether a row puts slots on a board or a PDF. `frequency` means one thing — an interval in minutes — and is null whenever there is no interval to state. Clients must read a null or unrecognised value as `SCHEDULED`, and must treat a null or non-positive `frequency` as "no recurrence" whatever the kind says.
Reserved: one unbroken administration, nothing discrete to place. Valid to receive so the enum never has to widen, but nothing produces it yet.
A single administration at `start`. `frequency` is always null.
Administered as needed. Never resolves to due or late. A PRN `frequency` is a **minimum interval** — "no more often than q4h" — and is display only today.
Recurs every `frequency` minutes from `start`. Counts toward overdue.