store_header is the only reader of a store file's leading bytes. It returns the header slot (a MessagePack str only, never bin, as in C#, TS and Python) and separately the proof that the bytes are one whole array, so a newer store cut short after its header is still named unsupported. leading_header, read_array_header and read_string are deleted. decode_store_file takes the header the reader already found.
Vectors bin-header-v1/v3/v9 join the readability manifest as refuses x4. Tests: a rename fault step (WriteStep::Rename) shows a failed rename is NotReplaced, the store intact and no staging file left; every merging writer refuses through a dangling link; the unsupported-format refusal names v1 and v3 and echoes only ASCII-digit headers.
Not yet compiled or run at this commit.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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