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/occultation_venus_20250919_regression_test.go

@@ -0,0 +1,38 @@
+package geojson_test
+
+import (
+	"testing"
+	"time"
+
+	"b612.me/astro/geojson"
+	"b612.me/astro/moon"
+)
+
+func TestMarshalPlanetOccultationVenus20250919OptimizedContoursAreStrictlyOrdered(t *testing.T) {
+	day := time.Date(2025, 9, 19, 0, 0, 0, 0, time.UTC)
+	paths, err := moon.FindPlanetOccultationPaths(
+		day.Add(-12*time.Hour), day.Add(36*time.Hour), moon.OccultationVenus,
+		moon.OccultationPathOptions{
+			Step:              5 * time.Minute,
+			TargetSpacingKM:   300,
+			DisableFootprints: true,
+			RiseSetStep:       30 * time.Second,
+		},
+	)
+	if err != nil || len(paths) != 1 {
+		t.Fatalf("FindPlanetOccultationPaths paths=%d err=%v, want one path", len(paths), err)
+	}
+	if len(paths[0].PartialBandContours) != 2 {
+		t.Fatalf("partial contours=%d, want two", len(paths[0].PartialBandContours))
+	}
+	for contourIndex, contour := range paths[0].PartialBandContours {
+		for index := 1; index < len(contour); index++ {
+			if !contour[index].Time.After(contour[index-1].Time) {
+				t.Fatalf("contour %d sample %d is not strictly increasing: %s then %s", contourIndex, index, contour[index-1].Time, contour[index].Time)
+			}
+		}
+	}
+	if _, err := geojson.MarshalPlanetOccultation(paths[0]); err != nil {
+		t.Fatalf("MarshalPlanetOccultation: %v", err)
+	}
+}