QLocale::system() uses short names of days and months for narrow formats

As documented as part of enum QLocale::FormatType, narrow format is treated as short when asking the system locale for day and month names.
This may be due to the MS-Win system APIs only providing full month names and "abbreviated" month names (that are in fact what would normally be termed short month names; typically three letters, rather than just one).
Even MS's APIs, however, distinguish full, short and abbreviated forms for day names; our generic Unix system back-end could perfectly well provide proper abbreviated forms for both month and day names; and the macOS APIs we use distinguish full, short and "very short" forms.
So there is no good reason not to support narrow formats just as faithfully as short and long, using relevant system APIs.

MS's deficiency can be documented as "we fall back to short when no narrow is available" but we should treat this as a deficiency of that back-end, only for months, rather than imposing it on all back-ends, both for months and for days.