fix(export): escape backslashes in tag table cells

A backslash right before a pipe, as in the tag name a\|b, turned the escaped
pipe into "\|": the table splitter reads "\\" as an escaped backslash and the
pipe as a column separator, shifting the row. Cells now escape '\' before '|'.
Recorded in PLAN 5.8.

YKanBan.Tests/Export/MarkdownExporterTests.cs

@@ -204,6 +204,29 @@ public class MarkdownExporterTests
         }
     }
 
+    [TestMethod]
+    public void TagTableCellsEscapeBackslashesSoAPipeAfterOneStaysEscaped()
+    {
+        CultureInfo? original = Resources.Culture;
+        try
+        {
+            Resources.Culture = new CultureInfo("en");
+            var tag = new TagModel(1, @"a\|b", RgbColor.Parse("#112233"), @"C:\temp\");
+            string markdown = MarkdownExporter.Export(
+                "W",
+                [new ColumnModel(1, "C", string.Empty, 1, 1)],
+                [],
+                [(tag, 0L)],
+                ExportedAt);
+
+            StringAssert.Contains(markdown, @"| a\\\|b | #112233 | C:\\temp\\ | 0 |");
+        }
+        finally
+        {
+            Resources.Culture = original;
+        }
+    }
+
     [TestMethod]
     [DataRow("plain", "`plain`")]
     [DataRow("a`b", "``a`b``")]

YKanBan/Export/MarkdownExporter.cs

@@ -11,7 +11,7 @@ namespace YKanBan.Export;
 /// uses that one per-language pattern. The whole document uses LF line endings
 /// on every platform. Card bodies and column descriptions are emitted verbatim
 /// apart from line endings; column and card titles are collapsed to one line;
-/// tag names become inline code with a long enough backtick fence; tag table cells escape '|' and turn line breaks into <br>. The
+/// tag names become inline code with a long enough backtick fence; tag table cells escape '\' and '|' and turn line breaks into <br>. The
 /// output is meant for reading only — there is no re-import.
 /// </summary>
 public static class MarkdownExporter
@@ -130,12 +130,14 @@ public static class MarkdownExporter
     }
 
     /// <summary>
-    /// Formats a table cell: escaped pipe characters, line breaks as &lt;br&gt;.
+    /// Formats a table cell: escaped backslashes and pipe characters, line breaks as &lt;br&gt;.
     /// </summary>
     /// <param name="text">The raw cell text.</param>
     /// <returns>The escaped single-line text.</returns>
     private static string Cell(string text) =>
-        text.Replace("|", "\\|").Replace("\r\n", "<br>").Replace("\r", "<br>").Replace("\n", "<br>");
+        // Backslashes first: the table splitter reads "\\" as an escaped backslash, so the
+        // '|' after a literal backslash would otherwise end the cell.
+        text.Replace("\\", "\\\\").Replace("|", "\\|").Replace("\r\n", "<br>").Replace("\r", "<br>").Replace("\n", "<br>");
 
     /// <summary>
     /// Renders text as an inline code span whose backtick fence is one longer than

docs/PLAN.md

@@ -246,7 +246,7 @@ card_tags: (card_id, tag_id) 复合主键，双向 ON DELETE CASCADE
 - 空列也保留（体现看板结构）；无标题卡片以 `### #61` 呈现
 - 转义规则：
   - 标签名以行内代码呈现，定界反引号数量 = 名称内最长连续反引号串长度 + 1（名称以反引号开头/结尾时两侧加空格）
-  - 标签总表单元格内 `|` → `\|`，换行 → `<br>`
+  - 标签总表单元格内 `\` → `\\`（先于 `|` 处理，否则 `a\|b` 中的 `|` 会被当成列分隔符），`|` → `\|`，换行 → `<br>`
   - 卡片正文与列 description **原样输出**（接受其中 Markdown 结构干扰文档层级，已接受）
 
 ### 5.9 主题（亮 / 暗 / 跟随系统）