feat: Sheet 性质判定（table/free_text/mixed）

tests/test_sheet_nature.py

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+from genesis.parsers.sheet_nature import SheetNature, classify_sheet, find_header_row
+
+
+def test_table_detection():
+    m = [["機能ID", "機能名"], ["A001", "社員登録"], ["A002", "退職処理"]]
+    assert classify_sheet(m) == SheetNature.TABLE
+
+
+def test_free_text_single_col():
+    m = [["新入社員を登録できる。"], ["氏名・所属・入社日を入力する。"]]
+    assert classify_sheet(m) == SheetNature.FREE_TEXT
+
+
+def test_free_text_many_empty_rows():
+    m = [["要求A"], [], [], [], ["要求B"]]
+    assert classify_sheet(m) == SheetNature.FREE_TEXT
+
+
+def test_header_row_index():
+    m = [["機能ID", "名前"], ["1", "田中"]]
+    assert find_header_row(m) == 0
+    assert find_header_row([["自由テキスト"]]) == -1