A one-dimensional physical quantity backed by a single canonical integer (bytes, nanoseconds, bytes/sec, ...). Implementing this is all a unit type needs to get +, -, and the ordering operators from Unit.Operators.

Implementations go through the type's own public constructor/accessor, so the opaque struct is read and rebuilt without ever being poked from the outside. Quantities are signed: subtraction may legitimately yield a negative value (a deficit), and nothing clamps it.