Schema validation library for sentry-options.
Schemas are loaded once into a [SchemaRegistry] and shared via Arc.
Values are validated against schemas as complete objects.
Values refresh lazily on read on the same thread. A read past
the staleness window re-reads the values directory on the calling thread
and publishes the new snapshot via ArcSwap (last-writer-wins). The full
scheme is in docs/architecture.md; the memory-ordering invariants are
documented inline in the refresh method below.