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

moon/svg/occultation_isochrone.go

@@ -0,0 +1,83 @@
+package svg
+
+import (
+	"fmt"
+	"html"
+	"strings"
+
+	"b612.me/astro/moon"
+)
+
+// writeOccultationGreatestTimeContours 绘制地方掩甚时刻等值线及其 HH:MM 标注。
+func writeOccultationGreatestTimeContours(
+	b *strings.Builder,
+	contours []moon.OccultationGreatestTimeContour,
+	layout starOccultationSVGLayout,
+	options StarOccultationSVGOptions,
+	placer *occultationLabelPlacer,
+) {
+	if len(contours) == 0 {
+		return
+	}
+	type isochroneLabel struct {
+		x, y float64
+		text string
+	}
+	labels := make([]isochroneLabel, 0, len(contours))
+	for _, contour := range contours {
+		for _, segment := range contour.Segments {
+			writeStarOccultationGeoLine(b, segment, layout,
+				occultationLineStyle{className: "greatest-time-isoline", color: "#1f6fb2", strokeWidth: 1.1})
+		}
+		text := contour.Time.In(options.Location).Format("15:04")
+		if x, y, ok := occultationGreatestTimeIsolineLabelPlacement(contour, text, layout, placer); ok {
+			labels = append(labels, isochroneLabel{x: x, y: y, text: text})
+		}
+	}
+	for _, label := range labels {
+		fmt.Fprintf(b, `<text class="greatest-time-isoline-label" x="%.3f" y="%.3f" fill="#1a5c96" stroke="#ffffff" stroke-width="2.6" paint-order="stroke" font-family="Arial, sans-serif" font-size="9" font-weight="700" text-anchor="middle">%s</text>`,
+			label.x, label.y, html.EscapeString(label.text))
+	}
+}
+
+// occultationGreatestTimeIsolineLabelPlacement 在最长的支路上取首个未被其它标签占用的标注点。
+func occultationGreatestTimeIsolineLabelPlacement(
+	contour moon.OccultationGreatestTimeContour,
+	text string,
+	layout starOccultationSVGLayout,
+	placer *occultationLabelPlacer,
+) (float64, float64, bool) {
+	longest := -1
+	for index, segment := range contour.Segments {
+		if longest < 0 || len(segment) > len(contour.Segments[longest]) {
+			longest = index
+		}
+	}
+	if longest < 0 {
+		return 0, 0, false
+	}
+	segment := contour.Segments[longest]
+	frame := layout.mapFrame()
+	middle := len(segment) / 2
+	for offset := 0; offset <= middle; offset++ {
+		for _, index := range []int{middle + offset, middle - offset} {
+			if index < 0 || index >= len(segment) {
+				continue
+			}
+			point := segment[index]
+			x, y, visible := frame.Project(point.Longitude, point.Latitude)
+			if !visible ||
+				x < frame.X+30 || x > frame.X+frame.Width-30 ||
+				y < frame.Y+18 || y > frame.Y+frame.Height-18 {
+				continue
+			}
+			// 优先压在等时线上方，其次下方；两者都被占用就换下一个候选点。
+			for _, baseline := range []float64{y - 6, y + 13} {
+				if placer.place(x, baseline, "middle", text, 9, 2) {
+					return x, baseline, true
+				}
+			}
+		}
+	}
+	return 0, 0, false
+}