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

barycentric_time_public_test.go

@@ -0,0 +1,69 @@
+package astro
+
+import (
+	"math"
+	"testing"
+	"time"
+
+	"b612.me/astro/basic"
+)
+
+// time.Time 往返经 JD 舍入，容差为 0.2 ms。
+func TestBarycentricTimeFacade(t *testing.T) {
+	tt := time.Date(2026, 1, 1, 0, 0, 0, 0, time.UTC)
+	if got := TCGMinusTT(tt); math.Abs(got-1.0777) > 5e-3 {
+		t.Errorf("TCG−TT=%v 秒, want ≈1.078", got)
+	}
+	if got := TCBMinusTT(tt); math.Abs(got-23.9757) > 5e-3 {
+		t.Errorf("TCB−TT=%v 秒, want ≈23.976", got)
+	}
+	if got := TDBMinusTT(tt); math.Abs(got) > 2e-3 {
+		t.Errorf("TDB−TT=%v 秒, want |·| ≤ 1.7 ms", got)
+	}
+	for _, tc := range []struct {
+		name string
+		fwd  func(time.Time) time.Time
+		back func(time.Time) time.Time
+	}{
+		{"TCG", TCGFromTT, TTFromTCG},
+		{"TCB", TCBFromTT, TTFromTCB},
+		{"TDB", TDBFromTT, TTFromTDB},
+	} {
+		round := tc.back(tc.fwd(tt))
+		if delta := math.Abs(round.Sub(tt).Seconds()); delta > 2e-4 {
+			t.Errorf("%s 往返差 %.6f 秒", tc.name, delta)
+		}
+		if forward := tc.fwd(tt); !forward.After(tt.Add(-time.Second)) || !forward.Before(tt.Add(time.Minute)) {
+			t.Errorf("%s 结果 %v 离开合理区间", tc.name, forward)
+		}
+	}
+	tcb := TCBFromTT(tt)
+	if diff := tcb.Sub(TDBFromTCB(tcb)).Seconds(); math.Abs(diff-23.9757) > 0.1 {
+		t.Errorf("TCB−TDB=%v 秒, want ≈23.976", diff)
+	}
+	if delta := math.Abs(TTFromTCB(tcb).Sub(tt).Seconds()); delta > 2e-4 {
+		t.Errorf("TT→TCB 往返差 %.6f 秒", delta)
+	}
+}
+
+func TestBarycentricTimeFacadePreservesScaleAndInstant(t *testing.T) {
+	tt := time.Date(2026, 1, 1, 0, 0, 0, 0, time.UTC)
+	for _, date := range []time.Time{tt, tt.In(time.FixedZone("UTC+08", 8*3600))} {
+		jd := basic.Date2JD(date.UTC())
+		for name, pair := range map[string][2]float64{
+			"TCG": {TCGMinusTT(date), basic.TCGMinusTTSeconds(jd)},
+			"TCB": {TCBMinusTT(date), basic.TCBMinusTTSeconds(jd)},
+			"TDB": {TDBMinusTT(date), basic.TDBMinusTTSeconds(jd)},
+		} {
+			if pair[0] != pair[1] {
+				t.Errorf("%s facade=%v, basic=%v", name, pair[0], pair[1])
+			}
+		}
+		if direct, composed := TDBFromTT(date), TDBFromTCB(TCBFromTT(date)); math.Abs(direct.Sub(composed).Seconds()) > 1e-4 {
+			t.Errorf("direct TDB=%v, composed=%v", direct, composed)
+		}
+		if got := TCGFromTT(date); got.Location() != time.UTC || !got.Equal(TCGFromTT(tt)) {
+			t.Errorf("TCG depends on input Location: %v", got)
+		}
+	}
+}