Function write_single_blob 

fn write_single_blob(
    ns: &str,
    tokens: &MuxBusTokens,
) -> Result<Vec<(String, PriorKeychainState)>, StoreError>

Write all three tokens as one combined JSON blob under a single keychain entry (blob_key — the CURRENT preferred format on macOS/Linux, see that constant’s doc comment). One SecItemAdd/ SecItemUpdate call is inherently atomic at the OS level, so this needs none of write_split_tokens’s cross-field generation-stamp/rollback machinery — there’s only one field. Returns the pre-call state wrapped in the same Vec<(String, PriorKeychainState)> shape write_split_tokens returns, so muxbus_save’s SQL-failure rollback branch works unchanged regardless of which one ran.