Refactored `detectDateOrder` in `src/engine/datetime.ts` to accept an optional `locale` argument. This allows `src/engine/datetime.test.ts` to directly verify actual Date-Month-Year and Month-Day-Year ordering using real native `Intl.DateTimeFormat` outputs for "en-GB" and "en-US", rather than faking the API output with spies. This vastly improves the realism and robustness of these tests.
Co-authored-by: Horizontal42 <75383519+Horizontal42@users.noreply.github.com>
Covers detectDateOrder, resolveZone, addToDate and isCalendarUnit —
none of it had direct unit coverage before, only indirectly through
the engine's date-arithmetic tests.
