test: 覆盖率提升至 100%（fail_under 90→99，13 个防御/边界用例）

tests/test_sheet_nature.py

@@ -29,4 +29,21 @@ def test_classify_sheet_mixed_with_bullet_line():
 
 def test_classify_sheet_mixed_with_square_line():
     m = [["ID", "名前"], ["1", "田中"], ["■備考行"]]
-    assert classify_sheet(m) == SheetNature.MIXED
+    assert classify_sheet(m) == SheetNature.MIXED
+
+
+def test_classify_empty_matrix_is_free_text():
+    # 空矩阵：_free_text_like 对空矩阵返回 True（L26 分支）
+    assert classify_sheet([]) == SheetNature.FREE_TEXT
+
+
+def test_classify_sparse_multicol_is_free_text():
+    # 多列但非空行占比 < 0.7 → FREE_TEXT（L32 稀疏分支）
+    m = [["ID", "名前"], [], [], [], [], ["1", "田中"]]
+    assert classify_sheet(m) == SheetNature.FREE_TEXT
+
+
+def test_classify_header_no_bullet_is_table():
+    # 表头有效且无 ・/■ 行 → 落入 TABLE（L40→44 无匹配循环路径）
+    m = [["ID", "名前"], ["1", "田中"], ["2", "佐藤"]]
+    assert classify_sheet(m) == SheetNature.TABLE