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

formula/distance.go

@@ -0,0 +1,22 @@
+package formula
+
+import "b612.me/astro/tools"
+
+// DistanceUnit 距离输入单位，与 tools 共用同一套枚举 / a unit accepted for distance input, shared with tools.
+type DistanceUnit = tools.DistanceUnit
+
+const (
+	// DistanceParsec 秒差距 / parsec.
+	DistanceParsec = tools.DistanceParsec
+	// DistanceLightYear 光年 / light-year.
+	DistanceLightYear = tools.DistanceLightYear
+	// DistanceAU 天文单位 / astronomical unit.
+	DistanceAU = tools.DistanceAU
+)
+
+// Distance 把给定单位的距离换算为秒差距 / converts a distance in the given unit to parsecs.
+//
+// 换算基元位于 tools，本函数只是对外转发；非正数、NaN 与未知单位返回 NaN。
+func Distance(value float64, unit DistanceUnit) float64 {
+	return tools.DistanceToParsecs(value, unit)
+}

formula/distance_test.go

@@ -0,0 +1,19 @@
+package formula
+
+import (
+	"math"
+	"testing"
+)
+
+// formula 只做对外转发，换算基元在 tools，这里只锁别名与转发不脱钩。
+func TestDistanceDelegatesToTools(t *testing.T) {
+	assertFormulaClose(t, "1 ly in pc", Distance(1, DistanceLightYear), 0.306601393786, 1e-12)
+	assertFormulaClose(t, "1 pc identity", Distance(2.5, DistanceParsec), 2.5, 1e-15)
+	assertFormulaClose(t, "aliases agree", float64(DistanceParsec), 0, 1e-15)
+	if got := Distance(0, DistanceParsec); !math.IsNaN(got) {
+		t.Fatalf("Distance(0) = %v, want NaN", got)
+	}
+	if got := Distance(1, DistanceUnit(200)); !math.IsNaN(got) {
+		t.Fatalf("Distance(1, unknown unit) = %v, want NaN", got)
+	}
+}