Cover the previously-untested RetCode::ERROR returns (term-buffer
overflow and malformed quoted term) and add a regression demonstrating
that a Saleae async-serial row with an invalid hex Value (e.g. "0xZZ")
is split cleanly by the parser -- which does no hex validation -- and is
rejected only by the subsequent hex_twochars_to_uint8() decode that
read_from_async_csv() relies on.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>