`SidebarGrouping` was a literal union kept in step with `SIDEBAR_GROUPING_ORDER`
by hand — a fifth grouping added to the type would compile while the cycle
silently skipped it. The array is now the single source (`as const`) and the
type is derived from it. The cycle test no longer pins the exact order (a
change-detector on the constant); it asserts one lap visits every grouping
once and returns to the start, and still fails when the cycle skips a step.
1450c7fcfb