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

internal/civiltime/event.go

@@ -0,0 +1,36 @@
+package civiltime
+
+import "time"
+
+// Event 合并变长民用日的固定偏移候选，handled=false 表示普通日期 / merges fixed-offset candidates on a civil day with an offset change.
+func Event(date time.Time, missing error, calculate func(time.Time) (time.Time, error)) (result time.Time, err error, handled bool) {
+	year, month, day := date.Date()
+	start := time.Date(year, month, day, 0, 0, 0, 0, date.Location())
+	end := time.Date(year, month, day+1, 0, 0, 0, 0, date.Location())
+	_, firstOffset := start.Zone()
+	_, lastOffset := end.Add(-time.Nanosecond).Zone()
+	if firstOffset == lastOffset {
+		return time.Time{}, nil, false
+	}
+	var firstErr error
+	for _, offset := range []int{firstOffset, lastOffset} {
+		fixed := time.Date(year, month, day, 0, 0, 0, 0, time.FixedZone("", offset))
+		candidate, candidateErr := calculate(fixed)
+		if candidateErr != nil {
+			if firstErr == nil {
+				firstErr = candidateErr
+			}
+			continue
+		}
+		if !candidate.Before(start) && candidate.Before(end) && (result.IsZero() || candidate.Before(result)) {
+			result = candidate.In(date.Location())
+		}
+	}
+	if !result.IsZero() {
+		return result, nil, true
+	}
+	if firstErr != nil {
+		return time.Time{}, firstErr, true
+	}
+	return time.Time{}, missing, true
+}