feat: 完善水星与金星全球凌日搜索及制图导出

- 基于平均下合周期与节点相位预筛候选，减少无效星历计算；跳过无法求解的单个会合周期，避免有限覆盖星历导致搜索提前终止
- 引入影轴 γ、半影半径及地球尺度几何判定，结合站心可见性识别地心无接触、仅部分地区可见的掠凌
- 添加金星 243 年、水星 46 年周期族编号，并明确编号适用范围及未分配时的零值语义
- 为 SVG、GeoJSON 和 KML 增加站心部分凌日可见区，适配接触等时线、几何元数据、图层样式及取景；为地心无接触凌日事件增加正射图，按可见区球面面积加权居中

basic/planet_transit_visibility.go

@@ -0,0 +1,79 @@
+package basic
+
+import "math"
+
+// PlanetTransitVisibilityColumn 子午线上的可见纬度区间，端点单位为度 / visible latitude intervals on a meridian, in degrees.
+type PlanetTransitVisibilityColumn struct {
+	Longitude float64
+	Intervals [][2]float64
+}
+
+// PlanetTransitVisibilityColumnsWithInstance 按 2° 经度、1° 纬度探测站心可见区并细化边界 / sample topocentric visibility at 2-degree longitude and 1-degree latitude spacing and refine boundaries.
+func PlanetTransitVisibilityColumnsWithInstance(transit PlanetTransitResult, inst *Instance) []PlanetTransitVisibilityColumn {
+	snapshot, ok := newPlanetTransitSnapshot(transit, inst)
+	if !ok {
+		return nil
+	}
+	snapshot.visibilityOnce.Do(func() {
+		for column := 0; column <= 180; column++ {
+			longitude := -180 + float64(column)*2
+			visible := func(latitude float64) bool {
+				local := snapshot.local(transit, Observer{Longitude: longitude, Latitude: latitude})
+				return local.Valid && local.Visible
+			}
+			value := PlanetTransitVisibilityColumn{Longitude: longitude}
+			previous, start := visible(-90), -90.0
+			for latitude := -89.0; latitude <= 90; latitude++ {
+				current := visible(latitude)
+				if current != previous {
+					left, right := latitude-1, latitude
+					for right-left > 0.0001 {
+						middle := (left + right) / 2
+						if visible(middle) == previous {
+							left = middle
+						} else {
+							right = middle
+						}
+					}
+					boundary := (left + right) / 2
+					if current {
+						start = boundary
+					} else {
+						value.Intervals = append(value.Intervals, [2]float64{start, boundary})
+					}
+				}
+				previous = current
+			}
+			if previous {
+				value.Intervals = append(value.Intervals, [2]float64{start, 90})
+			}
+			snapshot.visibility = append(snapshot.visibility, value)
+		}
+	})
+	result := make([]PlanetTransitVisibilityColumn, len(snapshot.visibility))
+	for index, column := range snapshot.visibility {
+		result[index] = PlanetTransitVisibilityColumn{Longitude: column.Longitude, Intervals: append([][2]float64(nil), column.Intervals...)}
+	}
+	return result
+}
+
+// PlanetTransitVisibilityPolygonsWithInstance 可见区的经度条带多边形 / longitude-strip polygons of the visible region.
+func PlanetTransitVisibilityPolygonsWithInstance(transit PlanetTransitResult, inst *Instance) [][]PlanetTransitIsochronePoint {
+	columns := PlanetTransitVisibilityColumnsWithInstance(transit, inst)
+	var result [][]PlanetTransitIsochronePoint
+	for index := 1; index < len(columns); index++ {
+		left, right := columns[index-1], columns[index]
+		for _, interval := range left.Intervals {
+			for _, other := range right.Intervals {
+				if math.Min(interval[1], other[1]) <= math.Max(interval[0], other[0]) {
+					continue
+				}
+				result = append(result, []PlanetTransitIsochronePoint{
+					{Longitude: left.Longitude, Latitude: interval[0]}, {Longitude: right.Longitude, Latitude: other[0]},
+					{Longitude: right.Longitude, Latitude: other[1]}, {Longitude: left.Longitude, Latitude: interval[1]},
+				})
+			}
+		}
+	}
+	return result
+}