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

geojson/example_test.go

@@ -0,0 +1,64 @@
+package geojson_test
+
+import (
+	"encoding/json"
+	"fmt"
+	"time"
+
+	"b612.me/astro/eclipse"
+	"b612.me/astro/geojson"
+)
+
+func ExampleMarshalSolarEclipse() {
+	date := time.Date(2024, time.April, 8, 0, 0, 0, 0, time.UTC)
+	partial, ok := eclipse.SolarEclipsePartialFootprints(
+		date,
+		eclipse.SolarEclipsePartialFootprintOptions{
+			Step:           10 * time.Minute,
+			BoundaryPoints: 180,
+		},
+	)
+	if !ok {
+		return
+	}
+	central, hasCentral := eclipse.SolarEclipseCentralPath(
+		date,
+		eclipse.SolarEclipsePathOptions{
+			Step:            time.Minute,
+			TargetSpacingKM: 20,
+		},
+	)
+	var centralPath *eclipse.SolarEclipsePath
+	if hasCentral {
+		centralPath = &central
+	}
+
+	data, err := geojson.MarshalSolarEclipse(partial, centralPath)
+	fmt.Println(err == nil, json.Valid(data))
+	// Output: true true
+}
+
+func ExampleMarshalSolarEclipseWithTimeMarkers() {
+	date := time.Date(2024, time.April, 8, 0, 0, 0, 0, time.UTC)
+	partial, ok := eclipse.SolarEclipsePartialFootprints(
+		date,
+		eclipse.SolarEclipsePartialFootprintOptions{Step: 20 * time.Minute, BoundaryPoints: 36},
+	)
+	if !ok {
+		return
+	}
+	central, ok := eclipse.SolarEclipseCentralPath(
+		date,
+		eclipse.SolarEclipsePathOptions{Step: 5 * time.Minute},
+	)
+	if !ok {
+		return
+	}
+	data, err := geojson.MarshalSolarEclipseWithTimeMarkers(
+		partial,
+		&central,
+		geojson.TimeMarkerOptions{Step: 30 * time.Minute, Location: time.FixedZone("CST", 8*60*60)},
+	)
+	fmt.Println(err == nil, json.Valid(data))
+	// Output: true true
+}