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

basic/solar_eclipse_15001121_regression_test.go

@@ -0,0 +1,20 @@
+package basic
+
+import "testing"
+
+func TestSolarEclipse15001121ShallowTotalBandCloses(t *testing.T) {
+	result := SolarEclipsePartialFootprints(
+		JDECalc(1500, 11, 21),
+		SolarEclipsePartialFootprintOptions{StepDays: 2.0 / 1440.0, BoundaryPoints: 96},
+	)
+	if result.Eclipse.Type != SolarEclipseTotal || result.Eclipse.Centrality != SolarEclipseCentralTwoLimits {
+		t.Fatalf("type=%s centrality=%s, want two-limit total eclipse", result.Eclipse.Type, result.Eclipse.Centrality)
+	}
+	if len(result.CentralBandSegments) != 1 {
+		t.Logf("footprints=%d closures=%d", len(result.CentralBandFootprints), len(result.CentralBandHorizonClosures))
+		t.Fatalf("central-band segments=%d, want one closed envelope", len(result.CentralBandSegments))
+	}
+	if len(result.CentralBandHorizonClosures) != 2 {
+		t.Fatalf("horizon closures=%d, want two", len(result.CentralBandHorizonClosures))
+	}
+}