ExactlyOneSeq

ExactlyOneSeq returns a shallow copy when exactly one source-ordered sequence value is accepted by predicate. It stops pulling at the second match. If input ends with no matches, it returns the zero value of T with ErrNoMatch; if it finds a second match, it returns the zero value of T with ErrMultipleMatches. Callers can classify either error with errors.Is. With an infinite input, ExactlyOneSeq returns only after a second match; it cannot confirm a no-match or exactly-one result without source exhaustion.