ExistingHeader read the whole snapshot and swallowed every failure into "no header": a
truncated v3 file got a v1 raw write and an unknown v9 file was overwritten, where
TypeScript, Python and Rust refuse. CultDocumentMessagePackSerialization.ReadStoreHeader
reads the prefix header, refuses a file that is not one complete MessagePack value or whose
format this runtime cannot read, and the raw write keeps a whole marked file marked.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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