The derives' slot visitors count the slots past the declared ones and note them into a scope the decode entry opens,
instead of skipping them silently; a unit arm with slots decodes and marks. Every write entry asks one check over the
held envelope, decoded under the mark, and refuses with SchemaConflictError.unread_slots before any store is touched.
Breaking for struct-literal construction of SchemaConflictError (0.3.x to 0.4.0).
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>