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/include.go

@@ -0,0 +1,73 @@
+package internal
+
+import (
+	"fmt"
+	"strings"
+)
+
+type IncludeResolver func(path string) (string, error)
+
+type IncludeManager struct {
+	resolver IncludeResolver
+	stack    []string
+}
+
+func NewIncludeManager(resolver IncludeResolver) *IncludeManager {
+	return &IncludeManager{resolver: resolver}
+}
+
+func (m *IncludeManager) Resolve(path string) (string, error) {
+	return m.resolveInternal(path, m.stack)
+}
+
+func (m *IncludeManager) resolveInternal(path string, stack []string) (string, error) {
+	for i, p := range stack {
+		if p == path {
+			return "", fmt.Errorf("circular include detected: %s is already in the include chain at depth %d", path, i)
+		}
+	}
+
+	src, err := m.resolver(path)
+	if err != nil {
+		return "", fmt.Errorf("failed to resolve include %q: %w", path, err)
+	}
+
+	newStack := make([]string, len(stack)+1)
+	copy(newStack, stack)
+	newStack[len(stack)] = path
+
+	return m.expandIncludes(src, newStack)
+}
+
+func (m *IncludeManager) expandIncludes(src string, stack []string) (string, error) {
+	result := src
+	offset := 0
+
+	for {
+		start := strings.Index(result[offset:], `@include("`)
+		if start < 0 {
+			break
+		}
+
+		absStart := offset + start
+		pathStart := absStart + len(`@include("`)
+
+		end := strings.Index(result[pathStart:], `")`)
+		if end < 0 {
+			break
+		}
+
+		includePath := result[pathStart : pathStart+end]
+		absEnd := pathStart + end + len(`")`)
+
+		resolved, err := m.resolveInternal(includePath, stack)
+		if err != nil {
+			return "", err
+		}
+
+		result = result[:absStart] + resolved + result[absEnd:]
+		offset = absStart + len(resolved)
+	}
+
+	return result, nil
+}