A distinct `const` does not make `oneOf` alternatives mutually exclusive
unless the property carrying it is required: two branches pinning an OPTIONAL
`kind` both match `{}`, so arguments omitting it satisfy more than one
alternative — exactly what `oneOf` forbids. The property must now be required
by the root or by every branch.
Also makes `propertiesOf` total (an unreadable `properties` reads as empty,
with a separate predicate for the one caller that must tell them apart), which
removes a row of unreachable `?? {}` fallbacks, and covers the value/type
comparisons the const checks rest on.
Signed-off-by: cliffhall <cliff@futurescale.com>
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