GetNextDerivationIndex passed over a vault whose vault-metadata.json could
not be read or parsed, so the next vault made from the same mnemonic could
get that vault's derivation index and so the same long-term key. It now
returns an error naming the file, so vault create and init stop with that
error and leave the state directory as it was. A vault directory with no
metadata file is still skipped.
The regression test breaks one vault's metadata, once with content that
does not parse and once with a test filesystem that fails to open it, and
requires CreateVault with the same mnemonic to fail, naming the file, and
leave no new vault.
Model: opus-5-5
sneak