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

basic/saturn_ring_ephemeris_instance.go

@@ -0,0 +1,110 @@
+package basic
+
+import (
+	"math"
+
+	. "b612.me/astro/tools"
+)
+
+func saturnRingSunLoBoWithInstance(jde float64, n int, node float64, inst *Instance) (lon, lat float64) {
+	lon = PlanetCoordinateNWithInstance(5, 0, jde, n, inst)
+	lat = PlanetCoordinateNWithInstance(5, 1, jde, n, inst)
+	distance := PlanetCoordinateNWithInstance(5, 2, jde, n, inst)
+	lat -= 0.000764 * Cos(lon-node) / distance
+	lon -= 0.01759 / distance
+	return lon, lat
+}
+
+func saturnRingParametersNWithInstance(jde float64, n int, inst *Instance) (earthLatitude, sunLatitude, positionAngle, deltaU, majorAxis, minorAxis float64) {
+	inclination, node := saturnRingPlane(jde)
+	earthLon, earthLat := SaturnApparentLoBoNWithInstance(jde, n, inst)
+	sunLon, sunLat := saturnRingSunLoBoWithInstance(jde, n, node, inst)
+	earthLatitude = saturnRingLatitude(inclination, node, earthLon, earthLat)
+	sunLatitude = saturnRingLatitude(inclination, node, sunLon, sunLat)
+	positionAngle = saturnRingPositionAngle(jde, inclination, node, earthLon, earthLat)
+	earthU := saturnRingLongitude(inclination, node, earthLon, earthLat)
+	sunU := saturnRingLongitude(inclination, node, sunLon, sunLat)
+	deltaU = saturnRingLongitudeDelta(earthU, sunU)
+	earthDistance := EarthSaturnAwayNWithInstance(jde, n, inst)
+	majorAxis = 375.35 / earthDistance
+	minorAxis = majorAxis * math.Abs(Sin(earthLatitude))
+	return
+}
+
+func saturnRingParametersWithInstance(jde float64, inst *Instance) (earthLatitude, sunLatitude, positionAngle, deltaU, majorAxis, minorAxis float64) {
+	return saturnRingParametersNWithInstance(jde, -1, inst)
+}
+
+// SaturnRingParametersWithInstance 土星环参数 / Saturn ring parameters with an explicit ephemeris instance.
+func SaturnRingParametersWithInstance(jde float64, inst *Instance) (earthLatitude, sunLatitude, positionAngle, deltaU, majorAxis, minorAxis float64) {
+	if inst == nil {
+		return SaturnRingParameters(jde)
+	}
+	return saturnRingParametersWithInstance(jde, inst)
+}
+
+// SaturnRingParametersNWithInstance 土星环参数（n 变体） / n-parameterised Saturn ring parameters with an explicit ephemeris instance.
+func SaturnRingParametersNWithInstance(jde float64, n int, inst *Instance) (earthLatitude, sunLatitude, positionAngle, deltaU, majorAxis, minorAxis float64) {
+	if inst == nil {
+		return SaturnRingParametersN(jde, n)
+	}
+	return saturnRingParametersNWithInstance(jde, n, inst)
+}
+
+// SaturnRingBWithInstance 土星环张角 / Saturn ring opening angle with an explicit ephemeris instance.
+func SaturnRingBWithInstance(jde float64, inst *Instance) float64 {
+	if inst == nil {
+		return SaturnRingB(jde)
+	}
+	earthLatitude, _, _, _, _, _ := saturnRingParametersWithInstance(jde, inst)
+	return earthLatitude
+}
+
+// SaturnRingSunBWithInstance 土星环太阳侧张角 / Sun-side Saturn ring opening angle with an explicit ephemeris instance.
+func SaturnRingSunBWithInstance(jde float64, inst *Instance) float64 {
+	if inst == nil {
+		return SaturnRingSunB(jde)
+	}
+	_, sunLatitude, _, _, _, _ := saturnRingParametersWithInstance(jde, inst)
+	return sunLatitude
+}
+
+// SaturnRingPositionAngleWithInstance 土星环位置角 / Saturn ring position angle with an explicit ephemeris instance.
+func SaturnRingPositionAngleWithInstance(jde float64, inst *Instance) float64 {
+	if inst == nil {
+		return SaturnRingPositionAngle(jde)
+	}
+	_, _, positionAngle, _, _, _ := saturnRingParametersWithInstance(jde, inst)
+	return positionAngle
+}
+
+// SaturnRingDeltaUWithInstance 土星环面经度差 / Saturn ring-plane longitude difference with an explicit ephemeris instance.
+func SaturnRingDeltaUWithInstance(jde float64, inst *Instance) float64 {
+	if inst == nil {
+		return SaturnRingDeltaU(jde)
+	}
+	_, _, _, deltaU, _, _ := saturnRingParametersWithInstance(jde, inst)
+	return deltaU
+}
+
+// SaturnRingAxisWithInstance 土星环外缘轴 / outer Saturn ring axes with an explicit ephemeris instance.
+func SaturnRingAxisWithInstance(jde float64, inst *Instance) (float64, float64) {
+	if inst == nil {
+		return SaturnRingAxis(jde)
+	}
+	_, _, _, _, majorAxis, minorAxis := saturnRingParametersWithInstance(jde, inst)
+	return majorAxis, minorAxis
+}
+
+// SaturnMagWithInstance 土星视星等 / apparent magnitude of Saturn with an explicit ephemeris instance.
+func SaturnMagWithInstance(jde float64, inst *Instance) float64 {
+	if inst == nil {
+		return SaturnMag(jde)
+	}
+	awaySun := SaturnRWithInstance(jde, inst)
+	awayEarth := EarthSaturnAwayWithInstance(jde, inst)
+	ringB, _, _, deltaU, _, _ := saturnRingParametersWithInstance(jde, inst)
+	ringB = math.Abs(ringB)
+	mag := -8.68 + 5*math.Log10(awaySun*awayEarth) + 0.044*deltaU - 2.6*Sin(ringB) + 1.25*Sin(ringB)*Sin(ringB)
+	return FloatRound(mag, 2)
+}