format_compound_unit adds an extra unit to preformatted string numerators #1398

On unchanged upstream e2a1791, format_compound_unit("1 kg", "kilometer", denominator_unit="hour", locale="en") returns "1 kg kilometer per hour"; calling the same function without numerator_unit returns "1 kg per hour". The documented contract says a string value is preformatted and its corresponding unit argument is ignored. The compound-unit optimization bypasses that rule for denominator_value == 1 and a known compound unit. This affects callers that accept either numeric quantities or already-localized display strings while retaining unit metadata. Eighteen regression cases across en/fi/fr and long/short/narrow lengths reproduce the mismatch before a production-code change. The numeric fast path should remain intact, but preformatted strings should use the existing general path. Identified and tested with AI assistance; no personal review or provenance attestation by the account owner is asserted.