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_test.go

@@ -1,8 +1,11 @@
 package basic
 
 import (
+	"fmt"
 	"math"
 	"testing"
+
+	"b612.me/astro/internal/ephem"
 )
 
 func TestSaturnRingParametersMeeusExample(t *testing.T) {
@@ -34,6 +37,53 @@ func TestSaturnRingNFullMatchesDefault(t *testing.T) {
 	assertSameFloat(t, "SaturnRingDeltaU", SaturnRingDeltaU(jd), SaturnRingDeltaUN(jd, -1))
 }
 
+func TestSaturnRingParametersNWithInstanceForwardsN(t *testing.T) {
+	previous := ephem.Active()
+	t.Cleanup(func() { ephem.SetActive(previous) })
+	jd := 2458850.5
+	names := []string{"EarthLatitude", "SunLatitude", "PositionAngle", "DeltaU", "MajorAxis", "MinorAxis"}
+	for _, model := range []ephem.Model{ephem.Legacy, ephem.Semianalytic} {
+		ephem.SetActive(model)
+		inst := ephem.NewModelInstance(model)
+		for _, n := range []int{-1, 0} {
+			expectedEarthLatitude, expectedSunLatitude, expectedPositionAngle, expectedDeltaU, expectedMajorAxis, expectedMinorAxis := SaturnRingParametersN(jd, n)
+			gotEarthLatitude, gotSunLatitude, gotPositionAngle, gotDeltaU, gotMajorAxis, gotMinorAxis := SaturnRingParametersNWithInstance(jd, n, inst)
+			got := []float64{gotEarthLatitude, gotSunLatitude, gotPositionAngle, gotDeltaU, gotMajorAxis, gotMinorAxis}
+			expected := []float64{expectedEarthLatitude, expectedSunLatitude, expectedPositionAngle, expectedDeltaU, expectedMajorAxis, expectedMinorAxis}
+			for index := range names {
+				assertClose(t, fmt.Sprintf("model=%d n=%d %s", model, n, names[index]), got[index], expected[index], 1e-10)
+			}
+		}
+	}
+}
+
+func TestSaturnRingParametersNWithInstanceIsolatedFromGlobalModel(t *testing.T) {
+	previous := ephem.Active()
+	t.Cleanup(func() { ephem.SetActive(previous) })
+	jd := 2458850.5
+	names := []string{"EarthLatitude", "SunLatitude", "PositionAngle", "DeltaU", "MajorAxis", "MinorAxis"}
+	for _, testCase := range []struct {
+		globalModel   ephem.Model
+		instanceModel ephem.Model
+	}{
+		{globalModel: ephem.Legacy, instanceModel: ephem.Semianalytic},
+		{globalModel: ephem.Semianalytic, instanceModel: ephem.Legacy},
+	} {
+		inst := ephem.NewModelInstance(testCase.instanceModel)
+		for _, n := range []int{-1, 0} {
+			ephem.SetActive(testCase.instanceModel)
+			expectedEarthLatitude, expectedSunLatitude, expectedPositionAngle, expectedDeltaU, expectedMajorAxis, expectedMinorAxis := SaturnRingParametersN(jd, n)
+			ephem.SetActive(testCase.globalModel)
+			gotEarthLatitude, gotSunLatitude, gotPositionAngle, gotDeltaU, gotMajorAxis, gotMinorAxis := SaturnRingParametersNWithInstance(jd, n, inst)
+			got := []float64{gotEarthLatitude, gotSunLatitude, gotPositionAngle, gotDeltaU, gotMajorAxis, gotMinorAxis}
+			expected := []float64{expectedEarthLatitude, expectedSunLatitude, expectedPositionAngle, expectedDeltaU, expectedMajorAxis, expectedMinorAxis}
+			for index := range names {
+				assertClose(t, fmt.Sprintf("global=%d instance=%d n=%d %s", testCase.globalModel, testCase.instanceModel, n, names[index]), got[index], expected[index], 1e-10)
+			}
+		}
+	}
+}
+
 func assertClose(t *testing.T, name string, got, want, tolerance float64) {
 	t.Helper()
 	if math.Abs(got-want) > tolerance {