refactor: remove implicit content sniffing and table conversion

- Split loadCSV into parseCSVString (content) and loadCSVFromPath
  (path); drop isCSV/looksLikeCsv heuristics
- Delete coerceSparkTables and markedTable label-header magic;
  plain markdown tables now render as plain tables
- Add explicit markdown role=spark-table fence syntax that converts
  pipe tables to CSV at scan time, with dice-header validation
- Map ESM-only github-slugger and csv-parse browser build to CJS in
  jest config; add content-registry tests

src/components/utils/csv-loader.ts

@@ -31,42 +31,6 @@ function parseFrontMatter(content: string): { frontmatter?: JSONObject; remainin
   }
 }
 
-/**
- * 检测字符串是否是 CSV 格式
- * @param str 待检测的字符串
- * @returns 如果是 CSV 格式返回 true
- */
-export function isCSV(str: string): boolean {
-  const trimmed = str.trim();
-  
-  // 检查是否以 YAML front matter 开头
-  if (trimmed.startsWith('---\n') || trimmed.startsWith('---\r\n')) {
-    return true;
-  }
-  
-  // 检查是否包含 CSV 特征：多行且有分隔符
-  const lines = trimmed.split(/\r?\n/).filter(line => line.trim() !== '');
-  if (lines.length < 2) {
-    return false;
-  }
-  
-  // 检测常见 CSV 分隔符
-  const separators = [',', '\t', ';', '|'];
-  const firstLine = lines[0];
-  
-  for (const sep of separators) {
-    if (firstLine.includes(sep)) {
-      // 检查其他行是否也有相同的分隔符
-      const hasSeparatorInOtherLines = lines.slice(1).some(line => line.includes(sep));
-      if (hasSeparatorInOtherLines) {
-        return true;
-      }
-    }
-  }
-  
-  return false;
-}
-
 /**
  * 解析 CSV 字符串
  * @template T 返回数据的类型，默认为 Record<string, string>
@@ -102,18 +66,12 @@ export function parseCSVString<T = Record<string, string>>(csvString: string, so
 /**
  * 加载 CSV 文件
  * @template T 返回数据的类型，默认为 Record<string, string>
- * @param pathOrContent 文件路径或 inline CSV 字符串
+ * @param path 文件路径（通过 file-index 获取内容）
  * @returns 解析后的 CSV 数据
  */
-export async function loadCSV<T = Record<string, string>>(pathOrContent: string): Promise<CSV<T>> {
-  // 检测是否是 inline CSV 数据
-  if (isCSV(pathOrContent)) {
-    return parseCSVString<T>(pathOrContent, 'inline');
-  }
-  
-  // 从索引获取文件内容
-  const content = await getIndexedData(pathOrContent);
-  return parseCSVString<T>(content, pathOrContent);
+export async function loadCSVFromPath<T = Record<string, string>>(path: string): Promise<CSV<T>> {
+  const content = await getIndexedData(path);
+  return parseCSVString<T>(content, path);
 }
 
 type JSONData = JSONArray | JSONObject | string | number | boolean | null;