CultStoreUnreadableException (a NotSupportedException) is the one refusal for a store file this runtime cannot read, thrown
by the single-file reader, the rewriting paths and CultMesh's reads, with the cause as the inner exception. The reader is exactly
one array of three slots (or an empty array): a header alone and an extra top-level slot are refused instead of read as empty or
skipped. CultMesh's older-catalog fallback is the same reader with the older entry reader, so it also requires one array.
Vectors: bad-body, header-only, missing-schema, variant-slot, extra-top-slot in the readability set, and a legacy-catalog store
with and without bytes after it for CultMesh. The persistence contract says what is true.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
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