feat: MergeHandler + TableExtractor（forward_fill / 表格提取）

tests/test_table_extractor.py

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+from genesis.data_models import CellValue, ExcelTable, SheetType
+from genesis.parsers.merge_fill import forward_fill
+from genesis.parsers.table_extractor import column_letter, extract_table
+
+
+def test_forward_fill_vertical():
+    matrix = [
+        ["機能ID", "機能名", "備考"],
+        ["A001", "", ""],
+        ["", "", "メモ"],
+    ]
+    filled = forward_fill(matrix, [(1, 2, 2, 2)])  # B1:B2 纵向合并
+    assert filled[1][1] == "機能名"
+    assert filled[2][2] == "メモ"
+
+
+def test_forward_fill_horizontal():
+    matrix = [["A", "B"], ["x", ""]]
+    filled = forward_fill(matrix, [(2, 1, 2, 2)])  # A2:B2 横向合并
+    assert filled[1][1] == "x"
+    assert filled[1][0] == "x"
+
+
+def test_column_letter():
+    assert column_letter(1) == "A"
+    assert column_letter(27) == "AA"
+
+
+def test_extract_table_basic():
+    matrix = [["ID", "名前"], ["1", "田中"], ["2", "佐藤"]]
+    table = extract_table("社員一覧", matrix, "f.xlsx", SheetType.FUNCTION)
+    assert isinstance(table, ExcelTable)
+    assert table.headers == ["ID", "名前"]
+    assert table.extraction_method == "openpyxl"
+    assert len(table.rows) == 2
+    first = table.rows[0]
+    assert isinstance(first["ID"], CellValue)
+    assert first["ID"].value == "1"
+    assert first["ID"].provenance.sheet_name == "社員一覧"
+    assert first["ID"].provenance.column == "A"