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

geojson/solar_isochrone_seam_test.go

@@ -0,0 +1,64 @@
+package geojson_test
+
+import (
+	"encoding/json"
+	"math"
+	"testing"
+	"time"
+
+	"b612.me/astro/eclipse"
+	"b612.me/astro/geojson"
+)
+
+// 契约：等时线整条支路共用同一时刻，跨反经线时必须照样在两侧补出接缝点——否则换日线处会留下缺口。
+func TestSolarEclipseIsochroneMeetsAtAntimeridian(t *testing.T) {
+	location := time.FixedZone("UTC+8", 8*3600)
+	date := time.Date(2009, time.July, 22, 0, 0, 0, 0, location)
+	level := time.Date(2009, time.July, 22, 11, 30, 0, 0, location) // 03:30 UT，正好跨反经线
+	partial, ok := eclipse.SolarEclipsePartialFootprintsNASABulletinSplitK(date,
+		eclipse.SolarEclipsePartialFootprintOptions{
+			Step:               20 * time.Minute,
+			BoundaryPoints:     24,
+			DisableRiseSet:     true,
+			GreatestTimeValues: []time.Time{level},
+		})
+	if !ok {
+		t.Fatal("expected the 2009-07-22 partial phase")
+	}
+	data, err := geojson.MarshalSolarEclipse(partial, nil)
+	if err != nil {
+		t.Fatalf("MarshalSolarEclipse: %v", err)
+	}
+	collection := decodeCollection(t, data)
+	lines := featuresWithRole(collection, "greatest-time-line")
+	if len(lines) != 1 {
+		t.Fatalf("greatest-time-line features=%d, want 1", len(lines))
+	}
+	var coordinates [][][]float64
+	if err := json.Unmarshal(lines[0].Geometry.Coordinates, &coordinates); err != nil {
+		t.Fatalf("decode coordinates: %v", err)
+	}
+	if len(coordinates) != 2 {
+		t.Fatalf("跨反经线的支路应拆成 2 段，得到 %d", len(coordinates))
+	}
+	east, west := coordinates[0], coordinates[1]
+	if len(east) < 2 || len(west) < 2 {
+		t.Fatal("段点数过少")
+	}
+	if last := east[len(east)-1]; math.Abs(last[0]+180) > 1e-9 {
+		t.Fatalf("东段末点经度 %v，应为 −180", last[0])
+	}
+	if first := west[0]; math.Abs(first[0]-180) > 1e-9 {
+		t.Fatalf("西段首点经度 %v，应为 +180", first[0])
+	}
+	if east[len(east)-1][1] != west[0][1] {
+		t.Fatalf("接缝两侧纬度不同：%v vs %v", east[len(east)-1][1], west[0][1])
+	}
+	for index, segment := range coordinates {
+		for point := 1; point < len(segment); point++ {
+			if math.Abs(segment[point][0]-segment[point-1][0]) > 180 {
+				t.Fatalf("段 %d 内仍有跨 180° 的边：%v → %v", index, segment[point-1], segment[point])
+			}
+		}
+	}
+}