新增: u-tpl SQL 模板引擎完整实现

- Lexer/Parser/Executor 三阶段架构
- #{param} 参数化 + ${raw} 原样替换 + 白名单安全策略
- @if/@for/@tpl/@include/@namespace 控制流
- 表达式引擎: 比较、逻辑、nil 检查、len() 内置函数
- 支持 ?/$1/:1 多数据库占位符风格
- 零依赖，纯 Go 标准库实现

internal/context.go

@@ -0,0 +1,68 @@
+package internal
+
+import (
+	"reflect"
+	"strings"
+)
+
+type Context struct {
+	vars map[string]any
+}
+
+func NewContext(vars map[string]any) *Context {
+	return &Context{vars: vars}
+}
+
+func (c *Context) Get(path string) (any, bool) {
+	if c.vars == nil {
+		return nil, false
+	}
+	return resolvePath(c.vars, path)
+}
+
+func resolvePath(current any, path string) (any, bool) {
+	for seg := range strings.SplitSeq(path, ".") {
+		if current == nil {
+			return nil, false
+		}
+		switch v := current.(type) {
+		case map[string]any:
+			var ok bool
+			current, ok = v[seg]
+			if !ok {
+				return nil, false
+			}
+		default:
+			val := reflect.ValueOf(current)
+			for val.Kind() == reflect.Pointer {
+				val = val.Elem()
+			}
+			if val.Kind() != reflect.Struct {
+				return nil, false
+			}
+			field := val.FieldByName(seg)
+			if !field.IsValid() {
+				field = findFieldIgnoreCase(val, seg)
+			}
+			if !field.IsValid() {
+				return nil, false
+			}
+			current = field.Interface()
+		}
+	}
+	return current, true
+}
+
+func findFieldIgnoreCase(v reflect.Value, name string) reflect.Value {
+	typ := v.Type()
+	for i := range typ.NumField() {
+		f := typ.Field(i)
+		if !f.IsExported() {
+			continue
+		}
+		if strings.EqualFold(f.Name, name) {
+			return v.Field(i)
+		}
+	}
+	return reflect.Value{}
+}