feat: 引入可切换星历模型并扩展天象观测链路

- 新增半解析、旧模型、自定义与多源堆叠星历实例，提供 Context、Instance、SPK 读取及状态与视位置计算
- 统一太阳、月球和行星实例接口，扩展黄经穿越、留、相位、最近角距、偕日可见、任意高度升落、轨道节点及凌日等天象查询
- 完善日月食、月掩与凌日的月缘剖面、有限盘面接触、站心修正、局地可见性、全球路径和地图输出
- 新增 SVG、GeoJSON、KML 天象输出及凌日、月掩和月缘地形相关缓存与几何求解
- 新增恒星星表导入与查询，支持 CSV、Hipparcos、BSC5、Stellarium 等格式及名称、交叉标识、锥面检索和星等过滤
- 补充 IAU 2006、地球定向、折射消光、大气观测、轨道传播及半解析行星/月球数据支持
- 将默认未来民用时标切换为 TimeScaleLeapHour，更新 2027–2400 农历速查表、中文日期解析和时区边界处理
- 增加事件基线、跨模型回归、几何边界、月缘地形和输出契约测试，完善中英文手册、示例资源、NOTICE 与忽略规则

astro.go

@@ -1,4 +1,4 @@
-// Package astro 提供进程级时标配置与时标换算 / process-wide time-scale configuration and conversions.
+// Package astro 提供高层星历上下文、模型切换与进程级时标配置 / high-level ephemeris contexts, model switching and process-wide time-scale configuration.
 package astro
 
 import "b612.me/astro/basic"
@@ -33,18 +33,18 @@ func DefaultTTMinusUTC() func(float64) float64 {
 	return basic.TTMinusUTCSecondsDefault
 }
 
-// TimeScaleFuturePolicy 民用时标换算政策，与输出用的 TimeScale 无关 / civil-time conversion policy, separate from output TimeScale.
+// TimeScaleFuturePolicy 民用时标换算政策，零值为闰时，与输出用的 TimeScale 无关 / civil-time conversion policy, leap hour as the zero value; separate from output TimeScale.
 type TimeScaleFuturePolicy = basic.TimeScaleFuturePolicy
 
 const (
-	// TimeScaleLeapSecond 默认按 ΔT 越限施加整数秒校正，不代表闰秒公告 / default integer-second corrections driven by extrapolated ΔT, not announcements.
+	// TimeScaleLeapHour 按 ΔT 越限施加整小时校正（2027 起 CGPM 决议） / applies hour-sized corrections as adopted by CGPM from 2027.
+	TimeScaleLeapHour = basic.TimeScaleLeapHour
+	// TimeScaleLeapSecond 按 ΔT 越限施加整数秒校正（1972–2026 确定值） / integer-second corrections (definitive 1972–2026).
 	TimeScaleLeapSecond = basic.TimeScaleLeapSecond
 	// TimeScaleAssumeUT1Tracking 窗口外固定末端 DUT1，平滑跟随 UT1 / holds the last observed DUT1 beyond the window.
 	TimeScaleAssumeUT1Tracking = basic.TimeScaleAssumeUT1Tracking
-	// TimeScaleFreezeUTCOffset 假设 UTC 偏移冻结在精确窗口末端 / freezes the UTC offset at the window end.
+	// TimeScaleFreezeUTCOffset 窗口外固定末端 TT−UTC / holds the last TT−UTC offset beyond the window.
 	TimeScaleFreezeUTCOffset = basic.TimeScaleFreezeUTCOffset
-	// TimeScaleLeapHour 按 ΔT 越限施加整小时校正，仅作情景演算 / applies hour-sized corrections as a scenario assumption.
-	TimeScaleLeapHour = basic.TimeScaleLeapHour
 	// TimeScaleUT1Civil 全时轴民用时标等同 UT1，TT−UTC 覆盖仍优先 / uses UT1 as civil time everywhere unless TT−UTC is overridden.
 	TimeScaleUT1Civil = basic.TimeScaleUT1Civil
 )