feat: 新增月掩与日月食地理绘图并提升观测计算精度

- 新增月掩恒星和行星：支持搜索、掩甚点、全球掩带及固定地点轨迹计算
- 支持恒星星表坐标转换、有限盘面行星接触事件和月掩 SVG 输出
- 新增日月食及月掩全球投影图、时间标记和 GeoJSON 地理数据接口
- 扩展日食中心线、南北界及偏食足迹采样，支持极区投影
- 修正站心时角、月出月落、月球视半径、折射和恒星自行计算
- 优化内外行星事件搜索、边界选择、极端输入处理和计算稳定性

internal/geodata/sphere_test.go

@@ -0,0 +1,33 @@
+package geodata
+
+import "testing"
+
+func TestJoinPolylineSegmentsUsesBothResultEndpoints(t *testing.T) {
+	segments := [][]GeoPoint{
+		{
+			{Longitude: -179, Latitude: 16},
+			{Longitude: -41, Latitude: 64},
+		},
+		{
+			{Longitude: 51, Latitude: 60},
+			{Longitude: 179, Latitude: 16},
+		},
+	}
+	joined := JoinPolylineSegments(segments)
+	if len(joined) != 4 {
+		t.Fatalf("joined point count = %d, want 4", len(joined))
+	}
+	if absoluteLongitude(joined[0].Longitude) > 90 || absoluteLongitude(joined[len(joined)-1].Longitude) > 90 {
+		t.Fatalf("joined open endpoints are on the antimeridian: first=%+v last=%+v", joined[0], joined[len(joined)-1])
+	}
+	if angularDistanceDegrees(joined[1], joined[2]) > 3 {
+		t.Fatalf("nearest antimeridian endpoints were not joined: %+v -> %+v", joined[1], joined[2])
+	}
+}
+
+func absoluteLongitude(value float64) float64 {
+	if value < 0 {
+		return -value
+	}
+	return value
+}