InvalidOperationException loading iconSet with formula cfvo from extLst

[swmal]

Description

Loading a workbook with an icon set conditional formatting rule stored in extLst throws when a threshold uses type="formula" with a numeric constant value.

Exception

System.InvalidOperationException: Value can only be changed if Type is
Num, Percent or Percentile. Current Type is "Formula"

Cause

Icon sets using extended sets (3Stars, 3Triangles, 5Boxes) or cross-sheet formulas are stored in extLst. The reader routes a numeric <xm:f> value to the Value setter without checking whether the cfvo type is Formula. The regular icon set reader and the databar reader already guard against this; the ext icon set reader does not.

Repro

using var package = new ExcelPackage();
var ws = package.Workbook.Worksheets.Add("Sheet1");
var iconSet = ws.ConditionalFormatting.AddThreeIconSet(
    new ExcelAddress("A1:A10"),
    eExcelconditionalFormatting3IconsSetType.Stars);
iconSet.Icon3.Type = eExcelConditionalFormattingValueObjectType.Formula;
iconSet.Icon3.Formula = "67";

var stream = new MemoryStream();
package.SaveAs(stream);
stream.Position = 0;
using var reloaded = new ExcelPackage(stream); // throws