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

internal/ephem/ephem.go

@@ -0,0 +1,160 @@
+// Package ephem 全局星历模型状态与实例接口 / global ephemeris model state and instance interfaces.
+package ephem
+
+import (
+	"sync"
+	"sync/atomic"
+)
+
+// Model 星历模型 / ephemeris model.
+type Model int32
+
+const (
+	// Legacy VSOP87 + ELP2000 截断级数 / truncated VSOP87 and ELP2000 series.
+	Legacy Model = iota
+	// Semianalytic 按 DE441 重拟的半解析表 / semi-analytic tables refitted to DE441.
+	Semianalytic
+	// Custom 外部 Engine 实例，不对应内置模型 / external Engine instance without a built-in model.
+	Custom Model = -1
+)
+
+var (
+	active                = int32(BuildDefault)
+	generation     uint64 = 1
+	modelEngines          = make(map[Model]func() Engine)
+	modelEnginesMu sync.RWMutex
+)
+
+// Active 当前模型 / the active model.
+func Active() Model { return Model(atomic.LoadInt32(&active)) }
+
+// Generation 模型切换世代，起始为 1 / model switch generation, starting at 1.
+func Generation() uint64 { return atomic.LoadUint64(&generation) }
+
+// SetActive 切换模型，返回是否发生变化 / switches the model and reports whether it changed.
+func SetActive(m Model) bool {
+	if m != Legacy && m != Semianalytic {
+		return false
+	}
+	globalInstanceMu.Lock()
+	defer globalInstanceMu.Unlock()
+	if atomic.SwapInt32(&active, int32(m)) == int32(m) {
+		return false
+	}
+	atomic.AddUint64(&generation, 1)
+	globalInstance.Store(NewModelInstance(m))
+	return true
+}
+
+// RegisterModelEngine 注册内置模型引擎工厂 / register a built-in model engine factory.
+func RegisterModelEngine(m Model, factory func() Engine) {
+	if m != Legacy && m != Semianalytic || factory == nil {
+		return
+	}
+	modelEnginesMu.Lock()
+	modelEngines[m] = factory
+	modelEnginesMu.Unlock()
+}
+
+// Instance 星历实例，可包含配置和缓存 / ephemeris instance with configuration and cache.
+type Instance struct {
+	model  Model
+	engine Engine
+}
+
+// NewInstance 创建自定义星历实例 / create an instance from a custom engine.
+func NewInstance(engine Engine) *Instance {
+	if IsNilEngine(engine) {
+		engine = nil
+	}
+	return &Instance{model: Custom, engine: engine}
+}
+
+// NewModelInstance 创建内置模型实例 / create an instance for a built-in model.
+func NewModelInstance(m Model) *Instance {
+	if m != Legacy && m != Semianalytic {
+		m = BuildDefault
+	}
+	modelEnginesMu.RLock()
+	factory := modelEngines[m]
+	modelEnginesMu.RUnlock()
+	var engine Engine
+	if factory != nil {
+		engine = factory()
+	}
+	return &Instance{model: m, engine: engine}
+}
+
+// Model 获取实例使用的模型 / get model used by this instance.
+func (inst *Instance) Model() Model {
+	if inst == nil {
+		return Custom
+	}
+	return inst.model
+}
+
+// Engine 获取底层星历引擎 / get the underlying ephemeris engine.
+func (inst *Instance) Engine() Engine {
+	if inst == nil {
+		return nil
+	}
+	return inst.engine
+}
+
+// State 计算目标相对中心的状态 / calculate target state relative to center.
+func (inst *Instance) State(target, center Body, jdTDB float64) (pos, vel [3]float64, err error) {
+	if inst == nil || inst.engine == nil {
+		return pos, vel, ErrNoEngine
+	}
+	return inst.engine.State(target, center, jdTDB)
+}
+
+// Coverage 返回目标覆盖范围 / return target coverage.
+func (inst *Instance) Coverage(target Body) (start, end float64, ok bool) {
+	if inst == nil || inst.engine == nil {
+		return 0, 0, false
+	}
+	return inst.engine.Coverage(target)
+}
+
+// Apparent 计算地心视方向 / calculate apparent geocentric direction.
+func (inst *Instance) Apparent(target Body, jdTT float64) (dir [3]float64, distKM, tauDays float64, err error) {
+	if inst == nil || inst.engine == nil {
+		return dir, 0, 0, ErrNoEngine
+	}
+	if engine, ok := inst.engine.(ApparentEngine); ok {
+		return engine.Apparent(target, jdTT)
+	}
+	// 只给几何状态的引擎（如 SPK）走通用矢量链。
+	return genericApparent(inst.engine, target, jdTT)
+}
+
+var (
+	globalInstance   atomic.Value
+	globalInstanceMu sync.Mutex
+)
+
+// Global 全局默认星历实例 / global default ephemeris instance.
+func Global() *Instance {
+	if inst, ok := globalInstance.Load().(*Instance); ok && inst != nil {
+		return inst
+	}
+	globalInstanceMu.Lock()
+	defer globalInstanceMu.Unlock()
+	if inst, ok := globalInstance.Load().(*Instance); ok && inst != nil {
+		return inst
+	}
+	inst := NewModelInstance(Active())
+	globalInstance.Store(inst)
+	return inst
+}
+
+// SetGlobal 设置全局默认星历实例 / set global default ephemeris instance.
+func SetGlobal(inst *Instance) {
+	if inst == nil {
+		return
+	}
+	globalInstanceMu.Lock()
+	defer globalInstanceMu.Unlock()
+	globalInstance.Store(inst)
+}