feat: 完善时标与天象几何计算并扩展输出接口

- 新增时标、ΔT 模型、质心时间与 UT1 支持
- 改进日月食、月掩、行星事件及路径边界计算
- 完善恒星三维自行与动态距离传播
- 扩展 SVG、GeoJSON、KML 输出与底层距离换算工具
- 整理中英文手册、示例资源及回归测试

tools/distance.go

@@ -0,0 +1,40 @@
+package tools
+
+import "math"
+
+// DistanceUnit 距离输入单位 / a unit accepted for distance input.
+type DistanceUnit uint8
+
+const (
+	// DistanceParsec 秒差距 / parsec.
+	DistanceParsec DistanceUnit = iota
+	// DistanceLightYear 光年 / light-year.
+	DistanceLightYear
+	// DistanceAU 天文单位 / astronomical unit.
+	DistanceAU
+)
+
+// 天文单位与光年取 IAU 定义值，秒差距由精确关系 648000/π 天文单位导出。
+const (
+	AstronomicalUnitKilometers = 149597870.7
+	LightYearKilometers        = 9460730472580.8
+	parsecAU                   = 648000 / math.Pi
+)
+
+// DistanceToParsecs 把给定单位的距离换算为秒差距 / converts a distance in the given unit to parsecs.
+//
+// 非正数、NaN 与未知单位返回 NaN。
+func DistanceToParsecs(value float64, unit DistanceUnit) float64 {
+	if math.IsNaN(value) || math.IsInf(value, 0) || value <= 0 {
+		return math.NaN()
+	}
+	switch unit {
+	case DistanceParsec:
+		return value
+	case DistanceLightYear:
+		return value * LightYearKilometers / AstronomicalUnitKilometers / parsecAU
+	case DistanceAU:
+		return value / parsecAU
+	}
+	return math.NaN()
+}

tools/distance_test.go

@@ -0,0 +1,49 @@
+package tools
+
+import (
+	"math"
+	"testing"
+)
+
+// 参考值由同一组 IAU 定义值独立推出，不使用 9 位凑整常数。
+const (
+	testPcPerLy = LightYearKilometers / AstronomicalUnitKilometers / parsecAU
+	testAuPerLy = LightYearKilometers / AstronomicalUnitKilometers
+)
+
+func assertDistanceClose(t *testing.T, name string, got, want, tolerance float64) {
+	t.Helper()
+	if math.Abs(got-want) > tolerance {
+		t.Fatalf("%s mismatch: got %.15f want %.15f", name, got, want)
+	}
+}
+
+func TestDistanceToParsecsUnits(t *testing.T) {
+	assertDistanceClose(t, "1 pc", DistanceToParsecs(1, DistanceParsec), 1, 1e-15)
+	assertDistanceClose(t, "1 ly in pc", DistanceToParsecs(1, DistanceLightYear), testPcPerLy, 1e-15)
+	assertDistanceClose(t, "1 ly in pc 参考 0.3066", testPcPerLy, 0.306601394, 1e-9)
+	assertDistanceClose(t, "1 pc in ly 参考 3.261563777", 1/testPcPerLy, 3.261563777, 1e-9)
+	assertDistanceClose(t, "1 ly in au 参考 63241.077084", DistanceToParsecs(1, DistanceLightYear)*parsecAU, 63241.077084, 1e-6)
+	assertDistanceClose(t, "1 pc in au 参考 206264.806247096", DistanceToParsecs(1, DistanceParsec)*parsecAU, 206264.806247096, 1e-9)
+	assertDistanceClose(t, "648000/pi au in pc", DistanceToParsecs(parsecAU, DistanceAU), 1, 1e-14)
+}
+
+func TestDistanceToParsecsRoundTrip(t *testing.T) {
+	for _, pc := range []float64{0.0001, 1, 2.637, 5.7803, 1000} {
+		assertDistanceClose(t, "pc round trip", DistanceToParsecs(pc, DistanceParsec), pc, 1e-15)
+		ly := pc / testPcPerLy
+		assertDistanceClose(t, "pc->ly->pc", DistanceToParsecs(ly, DistanceLightYear), pc, 1e-12)
+		assertDistanceClose(t, "au round trip", DistanceToParsecs(pc*parsecAU, DistanceAU), pc, 1e-12)
+	}
+}
+
+func TestDistanceToParsecsRejectsInvalidInput(t *testing.T) {
+	for _, value := range []float64{0, -1, math.NaN(), math.Inf(1), math.Inf(-1)} {
+		if got := DistanceToParsecs(value, DistanceParsec); !math.IsNaN(got) {
+			t.Fatalf("DistanceToParsecs(%v, parsec) = %v, want NaN", value, got)
+		}
+	}
+	if got := DistanceToParsecs(1, DistanceUnit(200)); !math.IsNaN(got) {
+		t.Fatalf("DistanceToParsecs(1, unknown unit) = %v, want NaN", got)
+	}
+}