feat(reader): legado 书源规则引擎 Go 移植 + 阅读 P0 后端

- internal/service/reader/rule/：逐方法移植 legado analyzeRule 包
  （AnalyzeRule/AnalyzeByJSoup/AnalyzeByJSonPath/AnalyzeByXPath/
  AnalyzeByRegex/AnalyzeUrl/RuleAnalyzer），JS 规则留 P2 接入点
- 书源导入（JSON数组/对象/Base64/URL）、多源聚合搜索（legado 四档
  排序合并）、详情/目录/正文（nextContentUrl 翻页合并）
- 数据模型 4 表注册迁移；/api/reader/* 路由组（书源/搜索/书架/进度/调试）
- 单测 + httptest 全链路端到端测试
- docs/reader-ui-spec.md：legado UI 交互仿制规格（供 P1 前端使用）

internal/service/reader/rule/xpath.go

@@ -0,0 +1,149 @@
+package rule
+
+import (
+	"strings"
+
+	"github.com/antchfx/htmlquery"
+	"golang.org/x/net/html"
+)
+
+// 本文件对应 AnalyzeByXPath.kt（JsoupXpath 语义，Go 侧用 antchfx/htmlquery）。
+
+type xpathAnalyzer struct {
+	root *html.Node
+}
+
+func newXPathAnalyzer(doc any) *xpathAnalyzer {
+	return &xpathAnalyzer{root: toHTMLNode(doc)}
+}
+
+// xpathResult 对应 getResult(xPath)。
+func (a *xpathAnalyzer) result(xPath string) []*html.Node {
+	if a.root == nil || xPath == "" {
+		return nil
+	}
+	return htmlquery.Find(a.root, xPath)
+}
+
+// xpathNodeString 对应 JXNode.asString()。
+func xpathNodeString(n *html.Node) string {
+	if n == nil {
+		return ""
+	}
+	if n.Type == html.TextNode {
+		return n.Data
+	}
+	if n.Type == html.ElementNode {
+		return htmlquery.InnerText(n)
+	}
+	if n.Data != "" {
+		return n.Data
+	}
+	return ""
+}
+
+// getElements 对应 AnalyzeByXPath.getElements。
+func (a *xpathAnalyzer) getElements(xPath string) []*html.Node {
+	if xPath == "" {
+		return nil
+	}
+	ra := NewRuleAnalyzer(xPath, false)
+	rules := ra.SplitRule("&&", "||", "%%")
+	if len(rules) == 1 {
+		return a.result(rules[0])
+	}
+	var out []*html.Node
+	var results [][]*html.Node
+	for _, rl := range rules {
+		temp := a.getElements(rl)
+		if len(temp) > 0 {
+			results = append(results, temp)
+			if ra.ElementsType() == "||" {
+				break
+			}
+		}
+	}
+	if len(results) > 0 {
+		if ra.ElementsType() == "%%" {
+			for i := 0; i < len(results[0]); i++ {
+				for _, temp := range results {
+					if i < len(temp) {
+						out = append(out, temp[i])
+					}
+				}
+			}
+		} else {
+			for _, temp := range results {
+				out = append(out, temp...)
+			}
+		}
+	}
+	return out
+}
+
+// getStringList 对应 AnalyzeByXPath.getStringList。
+func (a *xpathAnalyzer) getStringList(xPath string) []string {
+	var result []string
+	ra := NewRuleAnalyzer(xPath, false)
+	rules := ra.SplitRule("&&", "||", "%%")
+	if len(rules) == 1 {
+		for _, n := range a.result(xPath) {
+			result = append(result, xpathNodeString(n))
+		}
+		return result
+	}
+	var results [][]string
+	for _, rl := range rules {
+		temp := a.getStringList(rl)
+		if len(temp) > 0 {
+			results = append(results, temp)
+			if ra.ElementsType() == "||" {
+				break
+			}
+		}
+	}
+	if len(results) > 0 {
+		if ra.ElementsType() == "%%" {
+			for i := 0; i < len(results[0]); i++ {
+				for _, temp := range results {
+					if i < len(temp) {
+						result = append(result, temp[i])
+					}
+				}
+			}
+		} else {
+			for _, temp := range results {
+				result = append(result, temp...)
+			}
+		}
+	}
+	return result
+}
+
+// getString 对应 AnalyzeByXPath.getString：多节点以 \n 连接。
+func (a *xpathAnalyzer) getString(rule string) string {
+	ra := NewRuleAnalyzer(rule, false)
+	rules := ra.SplitRule("&&", "||")
+	if len(rules) == 1 {
+		nodes := a.result(rule)
+		if len(nodes) == 0 {
+			return ""
+		}
+		parts := make([]string, len(nodes))
+		for i, n := range nodes {
+			parts[i] = xpathNodeString(n)
+		}
+		return strings.Join(parts, "\n")
+	}
+	var textList []string
+	for _, rl := range rules {
+		temp := a.getString(rl)
+		if temp != "" {
+			textList = append(textList, temp)
+			if ra.ElementsType() == "||" {
+				break
+			}
+		}
+	}
+	return strings.Join(textList, "\n")
+}