feat(data): 实现 load_table 与 save_table

由用户本人实现（决策 25），Codex 只做审查与补测试。

- 按扩展名分派 .csv / .xlsx / .xls
- .csv 默认编码探测 utf-8 -> utf-8-sig -> gbk -> gb18030；
  显式传 encoding= 时不再探测
- na_values 默认吃下空串 / NA / N/A / #N/A / null / -
- save_table 自动建父目录，返回实际写入的 Path

src/mcm_toolkit/data/io.py

@@ -10,6 +10,8 @@ from __future__ import annotations
 from pathlib import Path
 from typing import Any
 
+import pandas as pd
+
 __all__ = ["load_table", "save_table"]
 
 
@@ -27,11 +29,41 @@ def load_table(path: str | Path, **kwargs: Any) -> Any:
     >>> df = load_table("data.csv")          # doctest: +SKIP
     >>> df.shape                             # doctest: +SKIP
     """
-    # TODO(L0): 编码探测 + 分派；Excel 缺 openpyxl 时给出可读报错。
-    raise NotImplementedError
+    path = Path(path)
+    suffix = path.suffix.lower()
+
+    encodings = ['utf-8','utf-8-sig','gbk','gb18030']
+    kwargs.setdefault('na_values',['','NA','N/A','#N/A','null','-'])
+
+    if suffix == ".csv":
+        if 'encoding' in kwargs:
+            return pd.read_csv(path,**kwargs)
+        for enc in encodings:
+            try:
+                return pd.read_csv(path,encoding=enc,**kwargs)
+            except UnicodeDecodeError:
+                continue
+            except FileNotFoundError:
+                raise FileNotFoundError(f"在这里找不到文件：{path}") from None
+        raise ValueError(f"无法解码文件，目前支持解码方式：{encodings}")
+    if suffix in ['.xlsx','.xls']:
+        return pd.read_excel(path,**kwargs)
+    raise ValueError(f"不支持的文件后缀：{suffix}")
 
 
-def save_table(df: Any, path: str | Path, index: bool = False, **kwargs: Any) -> Path:
+def save_table(df: pd.DataFrame, path: str | Path, index: bool = False, **kwargs: Any) -> Path:
     """保存 DataFrame，返回实际写入的路径。"""
-    # TODO(L0)
-    raise NotImplementedError
+    path = Path(path)
+    path.parent.mkdir(parents=True,exist_ok=True)
+    suffix = path.suffix.lower()
+
+    if not isinstance(df,pd.DataFrame):
+        raise TypeError(f"df 必须是一个pandas DataFrame,但是你上传了{type(df)}")
+
+    if suffix == '.csv':
+        df.to_csv(path,index=index,**kwargs)
+    elif suffix in ['.xlsx','.xls']:
+        df.to_excel(path,index=index,**kwargs)
+    else:
+        raise ValueError(f"不支持的文件后缀：{suffix}")
+    return path