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perf(Calendar): skip grid refreshes on hover and build month names once (#1395)

Hovering a cell re-ran the class refresh over the whole grid and, at its
end, the roving tab stop pass. Only the range preview depends on the
hovered date, and the preview needs an anchor: the start date while the
end is being picked, the end date otherwise. Without one, hover now only
fires `cellHover`. With one, it refreshes the classes but leaves the tab
stop alone, since hover never changes what is selected. The roving pass
moved out of `_updateClassNamesAndAriaLabels` to the click that changes
the selection and to the blur handlers, so focus leaving a cell or a
week row still puts the tab stop back on the selection, as it did when
the hover refresh ran on blur.

`getMonthsNames` builds one `Intl.DateTimeFormat` for the twelve names
instead of twelve `toLocaleString` calls, and a panel builds the month
list, the month details and the year list only for the view it shows.
An invalid `monthFormat` therefore fails when the months view renders
rather than at construction, the way an invalid `yearFormat` already
did.

Measured in Chromium with two panels, as script time around one
synchronous mouseover: a hover with nothing to preview goes from 0.73 to
0.02 ms and a hover with a start date picked from 0.60 to 0.43 ms. Twenty
page flips go from 47 to 37 ms, and the 21 renders of the run make no
`toLocaleString` call (504 before).

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