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What a setting holds, at runtime and as a declared default.
Two types rather than one, because they answer to different masters. Value is owned:
it comes from a file or an environment variable while the process runs, so it has to be
allocated. Const is what a declared default is, and every field of it is
const-constructible so a generated registry costs nothing to load — no parsing, no
allocation, nothing done per process start for a setting nobody reads.