Function findRetrogradePeriods

findRetrogradePeriods(
    body: CelestialBody,
    startJD: number,
    endJD: number,
): RetrogradePeriod[]
Find all retrograde periods for a body in a date range.
Parameters
- body: CelestialBody
  Celestial body
- startJD: number
  Start of search range
- endJD: number
  End of search range
Returns RetrogradePeriod[]
Array of retrograde periods
Remarks
Each retrograde period is defined by:
- Station Retrograde: When retrograde begins
- Station Direct: When direct motion resumes
Example
```
const periods = findRetrogradePeriods(CelestialBody.Mercury, jdStart, jdEnd);
for (const period of periods) {
  console.log(`Mercury Rx from ${period.stationRetroJD} to ${period.stationDirectJD}`);
  console.log(`Duration: ${period.durationDays.toFixed(1)} days`);
}
```
- Defined in transits/retrograde-transits.ts:258