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/solar_map_isochrone_test.go

@@ -0,0 +1,79 @@
+package svg
+
+import (
+	"strings"
+	"testing"
+	"time"
+
+	eclipsecore "b612.me/astro/eclipse"
+)
+
+func solarEclipseIsochroneMapOptions() SolarEclipseMapSVGOptions {
+	return SolarEclipseMapSVGOptions{
+		Width: 900, Height: 620, Location: time.UTC, PartialStep: 10 * time.Minute,
+		GreatestTimeStep: solarEclipseMapDefaultGreatestTimeStep,
+	}
+}
+
+// 等时线是可选图层：不请求时不得出现在输出里，请求时路径与标注都必须绘制。
+func TestSolarEclipseMapGreatestTimeIsochrones(t *testing.T) {
+	date := time.Date(2009, time.July, 22, 0, 0, 0, 0, time.UTC)
+	rendered, ok := SolarEclipseMapSVG(date, solarEclipseIsochroneMapOptions())
+	if !ok {
+		t.Fatal("missing map")
+	}
+	if !strings.Contains(rendered, `class="solar-greatest-time-isoline"`) {
+		t.Fatal("isochrone paths missing from a map that requested them")
+	}
+	if !strings.Contains(rendered, `class="solar-greatest-time-label"`) {
+		t.Fatal("isochrone labels missing from a map that requested them")
+	}
+
+	// 等时线是 opt-in：零值和负值都不画，与核心层和 moon/svg 一致；30 分钟只是 NASA 全球图的推荐间隔。
+	if got := normalizeSolarEclipseMapSVGOptions(date, SolarEclipseMapSVGOptions{}).GreatestTimeStep; got != 0 {
+		t.Fatalf("default isochrone step = %s, want them disabled", got)
+	}
+	if solarEclipseMapDefaultGreatestTimeStep != 30*time.Minute {
+		t.Fatalf("recommended NASA isochrone step = %s, want 30m", solarEclipseMapDefaultGreatestTimeStep)
+	}
+	for _, step := range []time.Duration{0, -1} {
+		disabled, ok := SolarEclipseMapSVG(date, SolarEclipseMapSVGOptions{
+			Width: 900, Height: 620, Location: time.UTC, PartialStep: 10 * time.Minute,
+			GreatestTimeStep: step,
+		})
+		if !ok {
+			t.Fatal("missing map")
+		}
+		if strings.Contains(disabled, "solar-greatest-time-isoline") ||
+			strings.Contains(disabled, "solar-greatest-time-label") {
+			t.Fatalf("step %s must not draw isochrones", step)
+		}
+	}
+	// 正值小于一分钟时夹取到一分钟，而不是关闭。
+	clamped := normalizeSolarEclipseMapSVGOptions(date, SolarEclipseMapSVGOptions{GreatestTimeStep: time.Second})
+	if clamped.GreatestTimeStep != time.Minute {
+		t.Fatalf("sub-minute isochrone step = %s, want it clamped to one minute", clamped.GreatestTimeStep)
+	}
+}
+
+// 电平必须落在偏食可见窗口内并按步长对齐，避免 59.999 秒那类截断。
+func TestSolarEclipseMapGreatestTimeLevelsAlignToStep(t *testing.T) {
+	date := time.Date(2009, time.July, 22, 0, 0, 0, 0, time.UTC)
+	options := normalizeSolarEclipseMapSVGOptions(date, solarEclipseIsochroneMapOptions())
+	info, ok := eclipsecore.SolarEclipseOnDateNASABulletinSplitK(date)
+	if !ok {
+		t.Fatal("missing eclipse")
+	}
+	levels := solarEclipseGreatestTimeLevels(info, options)
+	if len(levels) < 5 {
+		t.Fatalf("expected a full isochrone fan, got %d levels", len(levels))
+	}
+	for _, level := range levels {
+		if level.Second() != 0 || level.Nanosecond() != 0 || level.Minute()%30 != 0 {
+			t.Fatalf("level %s is not aligned to the requested step", level.Format("15:04:05.000"))
+		}
+		if level.Before(info.PartialBeginOnEarth) || level.After(info.PartialEndOnEarth) {
+			t.Fatalf("level %s is outside the partial-eclipse window", level.Format("15:04:05"))
+		}
+	}
+}