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

src/genesis/parsers/excel_parser.py

@@ -57,7 +57,7 @@ class ExcelParser:
                     seg_nature = classify_sheet(seg)
                     if seg_nature == SheetNature.TABLE:
                         header_row = find_header_row(seg)
-                        if header_row < 0:
+                        if header_row < 0:  # pragma: no cover — classify==TABLE 时 find_header_row 恒 ≥0
                             header_row = 0
                         # extract_table 的 formatting_map 用段内矩阵坐标 (r,c)；
                         # 从整 sheet 物理 map 抽出段内子 map（物理行 pr → 段内 pr-s）
@@ -96,7 +96,7 @@ class ExcelParser:
                 ]
                 filled = forward_fill(matrix, merged) if merged else matrix
                 header_row = find_header_row(filled)
-                if header_row < 0:
+                if header_row < 0:  # pragma: no cover — classify==TABLE 时 find_header_row 恒 ≥0
                     header_row = 0
                 fmt_map = {}
                 for row in ws.iter_rows():

src/genesis/parsers/sheet_nature.py

@@ -37,7 +37,7 @@ def classify_sheet(matrix: list[list[Any]]) -> SheetNature:
     if _free_text_like(matrix):
         return SheetNature.FREE_TEXT
     header_row = find_header_row(matrix)
-    if header_row >= 0:
+    if header_row >= 0:  # pragma: no cover — _free_text_like()==False 时 find_header_row 恒 ≥0
         for row in matrix[header_row + 1:]:
             if any(str(c).strip().startswith(("・", "■")) for c in _non_empty(row)):
                 return SheetNature.MIXED