Implement phase 5 critical branch extraction

src/RolemasterDb.ImportTool/Parsing/ColumnarCellEntry.cs

@@ -0,0 +1,10 @@
+namespace RolemasterDb.ImportTool.Parsing;
+
+internal sealed class ColumnarCellEntry(string? groupKey, string rollBandLabel, int rowIndex, string columnKey, List<string> lines)
+{
+    public string? GroupKey { get; } = groupKey;
+    public string RollBandLabel { get; } = rollBandLabel;
+    public int RowIndex { get; } = rowIndex;
+    public string ColumnKey { get; } = columnKey;
+    public List<string> Lines { get; } = lines;
+}