Implemented and merged in PR #103 (606cb57).
- Undated active and planned sprints now show
No scheduled datesusing their existing metadata styling. - Legacy start-only and end-only values render the available date; complete ranges are unchanged.
- Added active render coverage for no dates, start only, end only, and both dates, plus undated planned-sprint coverage.
Verification: make test passed, focused formatter coverage is 100%, and both GitHub CI jobs passed.