feat: 完善日月食与月掩几何链路并扩展历法接口

- 新增日月食中心带、偏食带、阴影足迹、等时线、食分线及升落边界计算，支持极区与混合食拓扑
- 新增日食单时刻阴影求解器、站心状态查询、批量采样和 ΔT 覆盖接口
- 重构恒星与行星月掩路径，补充有限盘面接触、站心修正、掩带宽度、极区投影及升落边界
- 扩展 SVG 与 GeoJSON 输出，支持详细面板、全球/极区/地球投影、边界闭合、时间标记和拓扑签名
- 扩展日月食候选搜索、局地搜索、沙罗序列预计算与范围外推，补充系列锚点和成员一致性校验
- 补齐古历纪年、儒略历独有闰日、多公历候选、历法改革跨日及精确日期运算接口
- 优化 ΔT、章动、恒星时、月球地平线、事件根搜索和本地星历缓存，降低重复计算开销并提升边界稳定

geojson/solar_eclipse_polar_envelope_test.go

@@ -0,0 +1,51 @@
+package geojson_test
+
+import (
+	"strings"
+	"testing"
+	"time"
+
+	"b612.me/astro/eclipse"
+	"b612.me/astro/geojson"
+	"b612.me/astro/internal/geodata"
+)
+
+func TestSolarPolarBandPreservesContinuousEnvelope(t *testing.T) {
+	for _, day := range [][3]int{
+		{2003, 11, 23}, {2021, 12, 4}, {2039, 12, 15}, {2061, 10, 13}, {2981, 10, 19},
+	} {
+		date := time.Date(day[0], time.Month(day[1]), day[2], 0, 0, 0, 0, time.UTC)
+		t.Run(date.Format("2006-01-02"), func(t *testing.T) {
+			partial, ok := eclipse.SolarEclipsePartialFootprints(date, eclipse.SolarEclipsePartialFootprintOptions{
+				Step: 2 * time.Minute, BoundaryPoints: 96, DisableRiseSet: true,
+			})
+			if !ok || len(partial.CentralBandSegments) != 1 {
+				t.Fatal("missing continuous envelope")
+			}
+			central, ok := eclipse.SolarEclipseCentralPath(date, eclipse.SolarEclipsePathOptions{Step: 2 * time.Minute, TargetSpacingKM: 700})
+			if !ok {
+				t.Fatal("missing central path")
+			}
+			for _, path := range []*eclipse.SolarEclipsePath{nil, &central} {
+				data, err := geojson.MarshalSolarEclipse(partial, path)
+				if err != nil {
+					t.Fatal(err)
+				}
+				band := featureWithRole(t, decodeCollection(t, data), "central-band")
+				source, _ := band.Properties["source"].(string)
+				if !strings.Contains(source, "envelope") {
+					t.Fatalf("continuous envelope replaced by %q", source)
+				}
+				rings := geoJSONMultiPolygonOuterRings(t, band)
+				var centers []geodata.GeoPoint
+				for _, point := range central.CenterLine {
+					centers = append(centers, geodata.GeoPoint{Longitude: point.Longitude, Latitude: point.Latitude})
+				}
+				if miss := geodata.SphericalPolygonsPathMissDistanceKM(rings, [][]geodata.GeoPoint{centers}, false); miss > 2 {
+					t.Fatalf("central band omits center line by %.3f km", miss)
+				}
+				assertClosedMultiPolygon(t, band)
+			}
+		})
+	}
+}