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

template.go

@@ -0,0 +1,85 @@
+package utpl
+
+import (
+	"errors"
+	"fmt"
+
+	"gitea.1216.top/lxy/u-tpl/internal"
+)
+
+type Result struct {
+	SQL  string
+	Args []any
+}
+
+type Template struct {
+	name      string
+	engine    *Engine
+	nodes     []internal.Node
+	blocks    map[string][]internal.Node
+	hasBlocks bool
+	namespace string
+}
+
+func (t *Template) Execute(vars map[string]any) (*Result, error) {
+	if t.hasBlocks {
+		return nil, &ExecError{
+			Pos:     Position{},
+			Message: fmt.Sprintf("template %q has named blocks, use ExecuteBlock instead", t.name),
+		}
+	}
+	return t.executeNodes(t.nodes, vars)
+}
+
+func (t *Template) ExecuteBlock(blockName string, vars map[string]any) (*Result, error) {
+	nodes, ok := t.blocks[blockName]
+	if !ok {
+		if t.namespace != "" {
+			nodes, ok = t.blocks[t.namespace+"."+blockName]
+		}
+	}
+	if !ok {
+		return nil, &ExecError{
+			Pos:     Position{},
+			Message: fmt.Sprintf("block %q not found in template %q", blockName, t.name),
+		}
+	}
+	return t.executeNodes(nodes, vars)
+}
+
+func (t *Template) ExecuteString(vars map[string]any) (string, error) {
+	result, err := t.Execute(vars)
+	if err != nil {
+		return "", err
+	}
+	return result.SQL, nil
+}
+
+func (t *Template) ExecuteBlockString(blockName string, vars map[string]any) (string, error) {
+	result, err := t.ExecuteBlock(blockName, vars)
+	if err != nil {
+		return "", err
+	}
+	return result.SQL, nil
+}
+
+func (t *Template) executeNodes(nodes []internal.Node, vars map[string]any) (*Result, error) {
+	executor := internal.NewExecutor(t.engine.style, t.engine.rawPolicy, t.engine.strict)
+	result, err := executor.Execute(nodes, vars)
+	if err != nil {
+		return nil, wrapExecError(err)
+	}
+	return &Result{SQL: result.SQL, Args: result.Args}, nil
+}
+
+func wrapExecError(err error) error {
+	var execErr *ExecError
+	if errors.As(err, &execErr) {
+		return err
+	}
+	var unsafeErr *UnsafeRawError
+	if errors.As(err, &unsafeErr) {
+		return err
+	}
+	return &ExecError{Message: err.Error()}
+}