feat: 完善时标与天象几何计算并扩展输出接口

- 新增时标、ΔT 模型、质心时间与 UT1 支持
- 改进日月食、月掩、行星事件及路径边界计算
- 完善恒星三维自行与动态距离传播
- 扩展 SVG、GeoJSON、KML 输出与底层距离换算工具
- 整理中英文手册、示例资源及回归测试

moon/svg/occultation_local_labels.go

@@ -0,0 +1,79 @@
+package svg
+
+import (
+	"fmt"
+	"html"
+	"math"
+	"strings"
+
+	"b612.me/astro/internal/svgchart"
+)
+
+// localOccultationOverviewLabel 是总览区一个接触标签：文本与它在图上的接触点位置。
+type localOccultationOverviewLabel struct {
+	text string
+	x, y float64
+}
+
+// writeLocalOccultationOverviewLabels 排布总览区的接触标签：先按原偏移试位，压叠时沿接触点径向
+// 外推、再上下错开，仍放不下就贴着月球外缘排成竖列；离接触点较远的标签用细引线连回去。
+func writeLocalOccultationOverviewLabels(
+	b *strings.Builder,
+	labels []localOccultationOverviewLabel,
+	table *svgchart.LabelTable,
+	cx, cy, moonRadius float64,
+	color string,
+) {
+	if len(labels) == 0 {
+		return
+	}
+	const fontSize = 11.0
+	spillTop := cy - moonRadius - 24
+	spill := 0
+	for _, item := range labels {
+		anchor, side := "start", 1.0
+		if item.x > cx {
+			anchor, side = "end", -1
+		}
+		baseY := item.y - 9
+		if item.y < cy-20 {
+			baseY = item.y + 15
+		}
+		candidates := []svgchart.LabelPlacement{{X: item.x + side*8, Y: baseY, Anchor: anchor}}
+		ux, uy := item.x-cx, item.y-cy
+		if norm := math.Hypot(ux, uy); norm > 1e-9 {
+			ux, uy = ux/norm, uy/norm
+		} else {
+			ux, uy = 0, -1
+		}
+		for step := 1; step <= 5; step++ {
+			reach := 12 + float64(step)*13
+			candidates = append(candidates,
+				svgchart.LabelPlacement{X: item.x + ux*reach + side*6, Y: item.y + uy*reach, Anchor: anchor},
+				svgchart.LabelPlacement{X: item.x + ux*reach - side*6, Y: item.y + uy*reach, Anchor: svgchart.MirrorLabelAnchor(anchor)},
+			)
+		}
+		for step := 1; step <= 4; step++ {
+			candidates = append(candidates,
+				svgchart.LabelPlacement{X: item.x + side*8, Y: baseY - float64(step)*15, Anchor: anchor},
+				svgchart.LabelPlacement{X: item.x + side*8, Y: baseY + float64(step)*15, Anchor: anchor},
+			)
+		}
+		placement, placed := table.Place(item.text, fontSize, candidates)
+		if !placed {
+			placement = svgchart.LabelPlacement{X: cx + side*(moonRadius+30), Y: spillTop + float64(spill)*15, Anchor: anchor}
+			for spill < 40 && table.OverlapsText(placement.X, placement.Y, fontSize, item.text, placement.Anchor) {
+				spill++
+				placement.Y = spillTop + float64(spill)*15
+			}
+			table.ReserveText(placement.X, placement.Y, fontSize, item.text, placement.Anchor)
+			spill++
+		}
+		if math.Hypot(placement.X-item.x, placement.Y-item.y) > 16 {
+			fmt.Fprintf(b, `<line x1="%.3f" y1="%.3f" x2="%.3f" y2="%.3f" stroke="%s" stroke-width="0.7" opacity="0.7"/>`,
+				item.x, item.y, placement.X, placement.Y, color)
+		}
+		fmt.Fprintf(b, `<text x="%.3f" y="%.3f" fill="%s" stroke="#ffffff" stroke-width="3" paint-order="stroke" font-family="Arial, sans-serif" font-size="%.0f" font-weight="700" text-anchor="%s">%s</text>`,
+			placement.X, placement.Y, color, fontSize, placement.Anchor, html.EscapeString(item.text))
+	}
+}