Track::fraction never needed this – the arithmetic is the same whatever
the numbers mean – which is exactly how the ruler came to assume minutes
and print 00:00 over a month. A renderer drawing an axis has to write a
label, and it cannot derive the unit from the numbers.
Added 2026-08-15, after a probe put a fifteen-day span on a
thirty-one-slot track and got correct geometry under a wall clock.
Non-exhaustive enums could have additional variants added in future. Therefore, when matching against variants of non-exhaustive enums, an extra wildcard arm must be added to account for any future variants.
Whole days. A month strip, a sprint, a stretch of leave.
A day-granularity axis is a strip, not a calendar: one line with
spans laid along it. What it deliberately does not do is wrap into
weeks, which is the shape that makes weekday periodicity visible and
the one job of a month grid that a strip cannot take over. See the
crate header.