feat: 完善日月食与月掩几何链路并扩展历法接口

- 新增日月食中心带、偏食带、阴影足迹、等时线、食分线及升落边界计算，支持极区与混合食拓扑
- 新增日食单时刻阴影求解器、站心状态查询、批量采样和 ΔT 覆盖接口
- 重构恒星与行星月掩路径，补充有限盘面接触、站心修正、掩带宽度、极区投影及升落边界
- 扩展 SVG 与 GeoJSON 输出，支持详细面板、全球/极区/地球投影、边界闭合、时间标记和拓扑签名
- 扩展日月食候选搜索、局地搜索、沙罗序列预计算与范围外推，补充系列锚点和成员一致性校验
- 补齐古历纪年、儒略历独有闰日、多公历候选、历法改革跨日及精确日期运算接口
- 优化 ΔT、章动、恒星时、月球地平线、事件根搜索和本地星历缓存，降低重复计算开销并提升边界稳定

basic/planet_apparent.go

@@ -60,15 +60,25 @@ func planetGeocentricPositionWithEarthN(planetIndex int, planetJD float64, ex, e
 }
 
 func planetApparentGeocentricPositionN(planetIndex int, jd float64, n int) (planetGeocentricPosition, float64) {
+	geo, tau, _ := planetApparentGeocentricPositionAndDistanceN(planetIndex, jd, n)
+	return geo, tau
+}
+
+func planetApparentGeocentricPositionAndDistanceN(
+	planetIndex int,
+	jd float64,
+	n int,
+) (planetGeocentricPosition, float64, float64) {
 	ex, ey, ez := earthHeliocentricXYZN(jd, n)
 	geoNow := planetGeocentricPositionWithEarthN(planetIndex, jd, ex, ey, ez, n)
-	tau := 0.0057755183 * math.Sqrt(geoNow.x*geoNow.x+geoNow.y*geoNow.y+geoNow.z*geoNow.z)
+	distance := math.Sqrt(geoNow.x*geoNow.x + geoNow.y*geoNow.y + geoNow.z*geoNow.z)
+	tau := 0.0057755183 * distance
 	geo := planetGeocentricPositionWithEarthN(planetIndex, jd-tau, ex, ey, ez, n)
 	baseLo := geo.lo
 	baseBo := geo.bo
 	geo.lo = Limit360(baseLo + GXCLo(baseLo, baseBo, jd)/3600.0 + Nutation2000Bi(jd))
 	geo.bo = baseBo + GXCBo(baseLo, baseBo, jd)/3600.0
-	return geo, tau
+	return geo, tau, distance
 }
 
 func planetTrueGeocentricPositionN(planetIndex int, jd float64, n int) (planetGeocentricPosition, float64) {