Enumerate every scalar Codable type (Int8 through Int64, Int, UInt
through UInt64, Float, Double, Bool, String) through both a keyed
container, via an all-scalars struct, and an unkeyed container, via
per-type arrays. This drives the keyed encode and decode container
methods and the encoder's scalar ladder to near-full coverage and pins
exact round-trip equality for each type.
