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

internal/ephem/ephem_test.go

@@ -0,0 +1,63 @@
+package ephem
+
+import "testing"
+
+type testEngine struct {
+	marker float64
+}
+
+func (engine testEngine) State(target, center Body, jdTDB float64) (pos, vel [3]float64, err error) {
+	pos[0] = engine.marker + float64(target-center)
+	vel[0] = jdTDB
+	return pos, vel, nil
+}
+
+func (engine testEngine) Coverage(target Body) (start, end float64, ok bool) {
+	return engine.marker, engine.marker + float64(target), true
+}
+
+func (engine testEngine) Apparent(target Body, jdTT float64) (dir [3]float64, distKM, tauDays float64, err error) {
+	dir[0] = engine.marker + float64(target)
+	return dir, jdTT, engine.marker, nil
+}
+
+func TestInstanceUsesItsOwnEngine(t *testing.T) {
+	first := NewInstance(testEngine{marker: 1})
+	second := NewInstance(testEngine{marker: 2})
+	firstPos, _, err := first.State(Mars, Earth, 2451545)
+	if err != nil {
+		t.Fatal(err)
+	}
+	secondPos, _, err := second.State(Mars, Earth, 2451545)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if firstPos[0] == secondPos[0] || first.Engine() == second.Engine() {
+		t.Fatal("instances are not isolated")
+	}
+	if got, want := first.Model(), Custom; got != want {
+		t.Fatalf("custom model = %v, want %v", got, want)
+	}
+}
+
+func TestInstanceApparentAndCoverage(t *testing.T) {
+	instance := NewInstance(testEngine{marker: 3})
+	start, end, ok := instance.Coverage(Mars)
+	if !ok || start != 3 || end != 7 {
+		t.Fatalf("coverage = %v, %v, %v", start, end, ok)
+	}
+	dir, distance, tau, err := instance.Apparent(Mars, 2451545)
+	if err != nil || dir[0] != 7 || distance != 2451545 || tau != 3 {
+		t.Fatalf("apparent = %v, %v, %v, %v", dir, distance, tau, err)
+	}
+}
+
+func TestInstanceWithoutEngineReportsExplicitErrors(t *testing.T) {
+	instance := &Instance{model: BuildDefault}
+	if _, _, err := instance.State(Mars, Earth, 2451545); err != ErrNoEngine {
+		t.Fatalf("State error = %v, want %v", err, ErrNoEngine)
+	}
+	if _, _, _, err := instance.Apparent(Mars, 2451545); err != ErrNoEngine {
+		t.Fatalf("Apparent error = %v, want %v", err, ErrNoEngine)
+	}
+}