feat: 扩展天文计算能力

- 新增日食、月食、本地可见性、中心线、半影区域、SVG 图示与沙罗周期信息
- 新增行星冲合、留、方照、物理星历、视直径、相位、亮肢角、轨道节点等计算
- 新增木星伽利略卫星位置、现象与接触事件计算
- 新增恒星星表、星座判定、自行修正与观测辅助能力
- 新增 coord、formula、orbit、sundial、lite/sun、lite/moon 等扩展包
- 完善农历年号、月相英文别名、视差角、大气质量、折射、日晷与双星计算
- 增加 NASA、JPL Horizons、IMCCE 等回归测试数据与基线测试
- 重构基础算法文件组织，补充大量公开 API 注释和语义回归测试
- 更新中文和英文 README，补充示例、精度说明、SVG 配图

orbit/phase_test.go

@@ -0,0 +1,80 @@
+package orbit
+
+import (
+	"math"
+	"testing"
+	"time"
+
+	"b612.me/astro/basic"
+)
+
+func TestDistancePhaseAndElongationHelpers(t *testing.T) {
+	elements := sampleObservationElements()
+	date := time.Date(2025, 11, 21, 20, 0, 0, 0, time.UTC)
+
+	sunDistance := SunDistance(date, elements)
+	earthDistance := EarthDistance(date, elements)
+	phaseAngle := PhaseAngle(date, elements)
+	illuminatedFraction := IlluminatedFraction(date, elements)
+	phase := Phase(date, elements)
+	elongation := Elongation(date, elements)
+
+	for name, value := range map[string]float64{
+		"sunDistance":         sunDistance,
+		"earthDistance":       earthDistance,
+		"phaseAngle":          phaseAngle,
+		"illuminatedFraction": illuminatedFraction,
+		"phase":               phase,
+		"elongation":          elongation,
+	} {
+		if math.IsNaN(value) || math.IsInf(value, 0) {
+			t.Fatalf("%s is not finite: %.18f", name, value)
+		}
+	}
+
+	if math.Abs(sunDistance-HeliocentricEclipticJ2000(date, elements).Distance) > 1e-12 {
+		t.Fatalf("sun distance mismatch: got %.15f want %.15f", sunDistance, HeliocentricEclipticJ2000(date, elements).Distance)
+	}
+	if math.Abs(earthDistance-GeocentricEclipticJ2000(date, elements).Distance) > 1e-12 {
+		t.Fatalf("earth distance mismatch: got %.15f want %.15f", earthDistance, GeocentricEclipticJ2000(date, elements).Distance)
+	}
+	if phaseAngle < 0 || phaseAngle > 180 {
+		t.Fatalf("phase angle out of range: %.12f", phaseAngle)
+	}
+	if elongation < 0 || elongation > 180 {
+		t.Fatalf("elongation out of range: %.12f", elongation)
+	}
+	if illuminatedFraction < 0 || illuminatedFraction > 1 {
+		t.Fatalf("illuminated fraction out of range: %.12f", illuminatedFraction)
+	}
+	if math.Abs(phase-illuminatedFraction) > 1e-12 {
+		t.Fatalf("phase alias mismatch: phase=%.15f illuminated=%.15f", phase, illuminatedFraction)
+	}
+
+	wantIlluminatedFraction := (1 + math.Cos(phaseAngle*math.Pi/180)) / 2
+	if math.Abs(illuminatedFraction-wantIlluminatedFraction) > 1e-12 {
+		t.Fatalf("illuminated fraction mismatch: got %.15f want %.15f", illuminatedFraction, wantIlluminatedFraction)
+	}
+
+	jd := ttJulianDay(date)
+	wantPhaseAngle := math.Acos(clampUnitLocal((sunDistance*sunDistance+earthDistance*earthDistance-basic.EarthAway(jd)*basic.EarthAway(jd))/(2*sunDistance*earthDistance))) * 180 / math.Pi
+	if math.Abs(phaseAngle-wantPhaseAngle) > 1e-12 {
+		t.Fatalf("phase angle mismatch: got %.15f want %.15f", phaseAngle, wantPhaseAngle)
+	}
+
+	objectLon, objectLat, _ := basic.OrbitApparentGeocentricEcliptic(jd, toBasicElements(elements))
+	wantElongation := basic.StarAngularSeparation(objectLon, objectLat, basic.HSunApparentLo(jd), basic.HSunTrueBo(jd))
+	if math.Abs(elongation-wantElongation) > 1e-12 {
+		t.Fatalf("elongation mismatch: got %.15f want %.15f", elongation, wantElongation)
+	}
+}
+
+func clampUnitLocal(value float64) float64 {
+	if value > 1 {
+		return 1
+	}
+	if value < -1 {
+		return -1
+	}
+	return value
+}