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

eclipse/svg/map_projection_contract_test.go

@@ -0,0 +1,39 @@
+package svg
+
+import (
+	"testing"
+
+	"b612.me/astro/internal/svgmap"
+)
+
+// 投影常量的取值字符串是与 moon/svg.MapProjection 共享的稳定契约。
+func TestEclipseMapProjectionValuesContract(t *testing.T) {
+	equivalents := []struct {
+		name  string
+		value EclipseMapProjection
+		text  string
+	}{
+		{name: "auto", value: EclipseMapProjectionAuto, text: ""},
+		{name: "equirectangular", value: EclipseMapProjectionEquirectangular, text: "equirectangular"},
+		{name: "north-polar", value: EclipseMapProjectionNorthPolar, text: "north-polar"},
+		{name: "south-polar", value: EclipseMapProjectionSouthPolar, text: "south-polar"},
+		{name: "orthographic", value: EclipseMapProjectionOrthographic, text: "orthographic"},
+	}
+	for _, testCase := range equivalents {
+		if got := string(testCase.value); got != testCase.text {
+			t.Fatalf("%s = %q, want %q", testCase.name, got, testCase.text)
+		}
+		if got := internalEclipseMapProjection(testCase.value); got != svgmap.Projection(testCase.text) {
+			t.Fatalf("%s maps to %q, want %q", testCase.name, got, svgmap.Projection(testCase.text))
+		}
+		if !validEclipseMapProjection(testCase.value) {
+			t.Fatalf("%s must pass validation", testCase.name)
+		}
+	}
+	// 边界：只有上表登记的字面量可以过校验，大小写与分隔符都不放宽。
+	for _, invalid := range []EclipseMapProjection{"auto", "Orthographic", "north_polar", "polar", "equirectangular "} {
+		if validEclipseMapProjection(invalid) {
+			t.Fatalf("validator accepted unsupported projection %q", invalid)
+		}
+	}
+}