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

internal/geodata/model.go

@@ -9,8 +9,24 @@ const (
 	ProjectionEquirectangular Projection = "equirectangular"
 	ProjectionNorthPolar      Projection = "north-polar"
 	ProjectionSouthPolar      Projection = "south-polar"
+	// ProjectionOrthographic 是正射（球面）投影，只画朝向视点的半个地球。
+	// ProjectionOrthographic is the orthographic (globe) projection, drawing only the hemisphere facing the view point.
+	ProjectionOrthographic Projection = "orthographic"
 )
 
+// ClipView 是裁剪所依赖的投影及其参数 / ClipView carries the projection and the parameters clipping depends on.
+type ClipView struct {
+	Projection Projection
+	// Center 是正射投影的视点，也就是可见半球的中心；其他投影忽略它。
+	// Center is the orthographic view point, the middle of the visible hemisphere; other projections ignore it.
+	Center GeoPoint
+}
+
+// Orthographic 判断视图是否为正射投影 / Orthographic reports whether the view uses the orthographic projection.
+func (view ClipView) Orthographic() bool {
+	return view.Projection == ProjectionOrthographic
+}
+
 // GeoPoint 是以度表示的地理点，东经为正 / GeoPoint is a geographic point in degrees, with east longitude positive.
 type GeoPoint struct {
 	Longitude float64