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
+}

internal/civiltime/event_test.go

@@ -0,0 +1,55 @@
+package civiltime
+
+import (
+	"errors"
+	"testing"
+	"time"
+)
+
+func TestEventFiltersByActualCivilDay(t *testing.T) {
+	loc, err := time.LoadLocation("America/New_York")
+	if err != nil {
+		t.Fatal(err)
+	}
+	missing := errors.New("no event on requested day")
+	cases := []struct {
+		month, day, offset, hour int
+		wantFound                bool
+	}{
+		{3, 8, -5, 23, false},
+		{11, 1, -5, 23, true},
+	}
+	for _, tc := range cases {
+		date := time.Date(2026, time.Month(tc.month), tc.day, 12, 0, 0, 0, loc)
+		candidate := time.Date(2026, time.Month(tc.month), tc.day, tc.hour, 30, 0, 0, time.FixedZone("", tc.offset*3600))
+		calls := 0
+		got, err, handled := Event(date, missing, func(fixed time.Time) (time.Time, error) {
+			calls++
+			if fixed.Location() == loc {
+				t.Fatal("calculation must use a fixed offset")
+			}
+			return candidate, nil
+		})
+		if !handled || calls != 2 {
+			t.Fatalf("handled=%t calls=%d", handled, calls)
+		}
+		if tc.wantFound {
+			if err != nil || !got.Equal(candidate) || got.Location() != loc {
+				t.Errorf("got %s, %v; want %s", got, err, candidate)
+			}
+		} else if !got.IsZero() || !errors.Is(err, missing) {
+			t.Errorf("outside-day result %s, %v", got, err)
+		}
+	}
+}
+
+func TestEventLeavesFixedOffsetToCaller(t *testing.T) {
+	date := time.Date(2026, 11, 1, 12, 0, 0, 0, time.FixedZone("EST", -5*3600))
+	_, _, handled := Event(date, nil, func(time.Time) (time.Time, error) {
+		t.Fatal("ordinary day invoked fallback")
+		return time.Time{}, nil
+	})
+	if handled {
+		t.Fatal("ordinary day reported handled")
+	}
+}