feat: 引入可切换星历模型并扩展天象观测链路

- 新增半解析、旧模型、自定义与多源堆叠星历实例，提供 Context、Instance、SPK 读取及状态与视位置计算
- 统一太阳、月球和行星实例接口，扩展黄经穿越、留、相位、最近角距、偕日可见、任意高度升落、轨道节点及凌日等天象查询
- 完善日月食、月掩与凌日的月缘剖面、有限盘面接触、站心修正、局地可见性、全球路径和地图输出
- 新增 SVG、GeoJSON、KML 天象输出及凌日、月掩和月缘地形相关缓存与几何求解
- 新增恒星星表导入与查询，支持 CSV、Hipparcos、BSC5、Stellarium 等格式及名称、交叉标识、锥面检索和星等过滤
- 补充 IAU 2006、地球定向、折射消光、大气观测、轨道传播及半解析行星/月球数据支持
- 将默认未来民用时标切换为 TimeScaleLeapHour，更新 2027–2400 农历速查表、中文日期解析和时区边界处理
- 增加事件基线、跨模型回归、几何边界、月缘地形和输出契约测试，完善中英文手册、示例资源、NOTICE 与忽略规则

geojson/spherical_sampling.go

@@ -69,7 +69,7 @@ func appendSphericalMapArcWithin(
 	// linear in lon/lat (notably the long horizon closure edges of shallow
 	// polar eclipses). The adaptive angular-error test alone cannot see that
 	// case and leaves visually abrupt 250+ km segments.
-	arcDistanceKM := solarCentralBandGeoPointDistanceKM(first, last)
+	arcDistanceKM := geodata.MeanDistanceKM(first, last)
 	if (sphericalMapChordError(first, middle, last) <= chordErrorDegrees && arcDistanceKM <= chordLimitKM) || depth >= 20 {
 		return append(points, first)
 	}
@@ -107,7 +107,7 @@ func sampleSphericalMapPath(source []pathSample) []pathSample {
 func sampleSphericalMapTriangle(points []geodata.GeoPoint) []geodata.GeoPoint {
 	apex, shortest := 0, math.Inf(1)
 	for i := range points {
-		if length := solarCentralBandGeoPointDistanceKM(points[(i+1)%3], points[(i+2)%3]); length < shortest {
+		if length := geodata.MeanDistanceKM(points[(i+1)%3], points[(i+2)%3]); length < shortest {
 			apex, shortest = i, length
 		}
 	}