Function nextCrossing

nextCrossing(
    value: (jdUt: number) => number,
    meanRatePerDay: number,
    targetDegrees: number,
    jdUtStart: number,
): number
First instant at or after jdUtStart when value reaches targetDegrees.

value must be increasing (mod 360) with a rate that stays within roughly ±25% of meanRatePerDay: the fixed-point iteration corrects at the mean rate, so each step shrinks the error geometrically. Converges to well under a second.
Parameters
- value: (jdUt: number) => number
  angle in degrees as a function of UT Julian day
- meanRatePerDay: number
  mean rate of value, degrees per day
- targetDegrees: number
  the angle to reach, degrees
- jdUtStart: number
  search start, UT Julian day
Returns number
Example
```
// First quarter moon after J2000: elongation reaches 90°.
nextCrossing(
  (jd) => normalizeDegrees(
    moonPosition(jd).apparentLongitude - sunPosition(jd).apparentLongitude,
  ),
  360 / 29.530588861,
  90,
  2451545,
); // ≈ 2451558.1 (2000-01-14 13:34 UT)
```
- Defined in core/src/math/crossing.ts:38