fix(xlsx): emit Excel 2010+ x14 extension for databar CF

Databar conditional format previously emitted only the Excel 2007
<x:dataBar> element, which causes Excel to render negative values as
positive (rightward bars, no zero-axis, no red for negatives).

Now emit the paired x14 extension under the worksheet extLst:
 - x14:dataBar with axisPosition, fillColor, negativeFillColor, axisColor
 - x14:cfRule linked to the 2007 cfRule via {B025F937-...} extension GUID
 - mc:Ignorable="x14" on the worksheet root (via new helper
   EnsureWorksheetX14Ignorable, factored out of the sparkline path)

New user props:
 - negativeColor=<hex>   (default FF0000)
 - axisColor=<hex>       (default 000000)
 - axisPosition=automatic|middle|none

Also: omit cfvo val="" attribute when type is min/max (Excel rejects
the empty attribute).

src/officecli/Handlers/Excel/ExcelHandler.Add.cs

@@ -1076,16 +1076,19 @@ public partial class ExcelHandler
                     Priority = NextCfPriority(GetSheet(cfWorksheet))
                 };
                 var dataBar = new DataBar();
-                dataBar.Append(new ConditionalFormatValueObject
+                // R10-1: when cfvo type is min/max, omit `val` attribute (Excel rejects val="").
+                var dbMinCfvo = new ConditionalFormatValueObject
                 {
-                    Type = minVal != null ? ConditionalFormatValueObjectValues.Number : ConditionalFormatValueObjectValues.Min,
-                    Val = minVal
-                });
-                dataBar.Append(new ConditionalFormatValueObject
+                    Type = minVal != null ? ConditionalFormatValueObjectValues.Number : ConditionalFormatValueObjectValues.Min
+                };
+                if (minVal != null) dbMinCfvo.Val = minVal;
+                dataBar.Append(dbMinCfvo);
+                var dbMaxCfvo = new ConditionalFormatValueObject
                 {
-                    Type = maxVal != null ? ConditionalFormatValueObjectValues.Number : ConditionalFormatValueObjectValues.Max,
-                    Val = maxVal
-                });
+                    Type = maxVal != null ? ConditionalFormatValueObjectValues.Number : ConditionalFormatValueObjectValues.Max
+                };
+                if (maxVal != null) dbMaxCfvo.Val = maxVal;
+                dataBar.Append(dbMaxCfvo);
                 dataBar.Append(new DocumentFormat.OpenXml.Spreadsheet.Color { Rgb = normalizedColor });
                 cfRule.Append(dataBar);
                 // CF6 — dataBar `showValue=false` hides the cell's numeric
@@ -1095,6 +1098,23 @@ public partial class ExcelHandler
                     dataBar.ShowValue = false;
                 ApplyStopIfTrue(cfRule, properties);
 
+                // R10-1: Also emit Excel 2010+ x14 extension so negative values
+                // render leftward in red with an axis. Without this block, Excel
+                // uses the 2007 dataBar which treats all values as positive
+                // (rightward blue bars, no axis, no red for negatives).
+                var dbGuid = "{" + Guid.NewGuid().ToString().ToUpperInvariant() + "}";
+                // Attach x14:id extension onto the 2007 cfRule so it's paired
+                // with the sibling x14:cfRule in the worksheet extLst.
+                var dbRuleExtList = new ConditionalFormattingRuleExtensionList();
+                var dbRuleExt = new ConditionalFormattingRuleExtension
+                {
+                    Uri = "{B025F937-C7B1-47D3-B67F-A62EFF666E3E}"
+                };
+                dbRuleExt.AddNamespaceDeclaration("x14", "http://schemas.microsoft.com/office/spreadsheetml/2009/9/main");
+                dbRuleExt.Append(new X14.Id(dbGuid));
+                dbRuleExtList.Append(dbRuleExt);
+                cfRule.Append(dbRuleExtList);
+
                 var cf = new ConditionalFormatting(cfRule)
                 {
                     SequenceOfReferences = new ListValue<StringValue>(
@@ -1104,6 +1124,57 @@ public partial class ExcelHandler
                 var wsElement = GetSheet(cfWorksheet);
                 InsertConditionalFormatting(wsElement, cf);
 
+                // R10-1: Build the x14:dataBar counterpart under worksheet extLst.
+                var dbNegColor = ParseHelpers.NormalizeArgbColor(properties.GetValueOrDefault("negativeColor", "FF0000"));
+                var dbAxisColor = ParseHelpers.NormalizeArgbColor(properties.GetValueOrDefault("axisColor", "000000"));
+                var dbAxisPos = (properties.GetValueOrDefault("axisPosition") ?? "automatic").ToLowerInvariant();
+                var dbAxisPosVal = dbAxisPos switch
+                {
+                    "middle" => X14.DataBarAxisPositionValues.Middle,
+                    "none" => X14.DataBarAxisPositionValues.None,
+                    _ => X14.DataBarAxisPositionValues.Automatic
+                };
+
+                var x14DataBar = new X14.DataBar
+                {
+                    MinLength = 0U,
+                    MaxLength = 100U,
+                    AxisPosition = dbAxisPosVal
+                };
+                var x14MinCfvo = new X14.ConditionalFormattingValueObject
+                {
+                    Type = minVal != null
+                        ? X14.ConditionalFormattingValueObjectTypeValues.Numeric
+                        : X14.ConditionalFormattingValueObjectTypeValues.AutoMin
+                };
+                if (minVal != null) x14MinCfvo.Append(new DocumentFormat.OpenXml.Office.Excel.Formula(minVal));
+                x14DataBar.Append(x14MinCfvo);
+                var x14MaxCfvo = new X14.ConditionalFormattingValueObject
+                {
+                    Type = maxVal != null
+                        ? X14.ConditionalFormattingValueObjectTypeValues.Numeric
+                        : X14.ConditionalFormattingValueObjectTypeValues.AutoMax
+                };
+                if (maxVal != null) x14MaxCfvo.Append(new DocumentFormat.OpenXml.Office.Excel.Formula(maxVal));
+                x14DataBar.Append(x14MaxCfvo);
+                x14DataBar.Append(new X14.FillColor { Rgb = normalizedColor });
+                x14DataBar.Append(new X14.NegativeFillColor { Rgb = dbNegColor });
+                x14DataBar.Append(new X14.BarAxisColor { Rgb = dbAxisColor });
+
+                var x14CfRule = new X14.ConditionalFormattingRule
+                {
+                    Type = ConditionalFormatValues.DataBar,
+                    Id = dbGuid
+                };
+                x14CfRule.Append(x14DataBar);
+
+                var x14Cf = new X14.ConditionalFormatting();
+                x14Cf.AddNamespaceDeclaration("xm", "http://schemas.microsoft.com/office/excel/2006/main");
+                x14Cf.Append(x14CfRule);
+                x14Cf.Append(new DocumentFormat.OpenXml.Office.Excel.ReferenceSequence(sqref));
+
+                EnsureWorksheetX14ConditionalFormatting(wsElement, x14Cf);
+
                 SaveWorksheet(cfWorksheet);
                 var dbCfCount = wsElement.Elements<ConditionalFormatting>().Count();
                 return $"/{cfSheetName}/cf[{dbCfCount}]";

src/officecli/Handlers/Excel/ExcelHandler.Helpers.cs

@@ -633,6 +633,61 @@ public partial class ExcelHandler
             rule.StopIfTrue = true;
     }
 
+    /// <summary>
+    /// Ensure the worksheet root declares `xmlns:x14` + `mc:Ignorable="x14"`.
+    /// Without both, Excel silently drops the x14 extension block where
+    /// sparklines, dataBar 2010+ extensions, and other Office2010 features
+    /// live. CONSISTENCY(x14-ignorable): same pattern the sparkline branch
+    /// uses inline.
+    /// </summary>
+    internal static void EnsureWorksheetX14Ignorable(Worksheet ws)
+    {
+        const string mcNs = "http://schemas.openxmlformats.org/markup-compatibility/2006";
+        const string x14Ns = "http://schemas.microsoft.com/office/spreadsheetml/2009/9/main";
+        if (ws.LookupNamespace("mc") == null)
+            ws.AddNamespaceDeclaration("mc", mcNs);
+        if (ws.LookupNamespace("x14") == null)
+            ws.AddNamespaceDeclaration("x14", x14Ns);
+        var ignorable = ws.MCAttributes?.Ignorable?.Value ?? "";
+        if (!ignorable.Split(' ').Contains("x14"))
+        {
+            ws.MCAttributes ??= new MarkupCompatibilityAttributes();
+            ws.MCAttributes.Ignorable = string.IsNullOrEmpty(ignorable) ? "x14" : $"{ignorable} x14";
+        }
+    }
+
+    /// <summary>
+    /// Append an x14:conditionalFormatting block to the worksheet's extLst under
+    /// ext URI `{78C0D931-6437-407d-A8EE-F0AAD7539E65}`. Creates the extension
+    /// on first call, appends to the existing x14:conditionalFormattings
+    /// container on subsequent calls. Also ensures mc:Ignorable="x14" is set.
+    /// </summary>
+    internal static void EnsureWorksheetX14ConditionalFormatting(Worksheet ws, X14.ConditionalFormatting x14Cf)
+    {
+        const string cfExtUri = "{78C0D931-6437-407d-A8EE-F0AAD7539E65}";
+        const string x14Ns = "http://schemas.microsoft.com/office/spreadsheetml/2009/9/main";
+
+        EnsureWorksheetX14Ignorable(ws);
+
+        var extList = ws.GetFirstChild<WorksheetExtensionList>() ?? ws.AppendChild(new WorksheetExtensionList());
+        var ext = extList.Elements<WorksheetExtension>().FirstOrDefault(e => e.Uri == cfExtUri);
+        X14.ConditionalFormattings cfContainer;
+        if (ext != null)
+        {
+            cfContainer = ext.GetFirstChild<X14.ConditionalFormattings>()
+                ?? ext.AppendChild(new X14.ConditionalFormattings());
+        }
+        else
+        {
+            ext = new WorksheetExtension { Uri = cfExtUri };
+            ext.AddNamespaceDeclaration("x14", x14Ns);
+            cfContainer = new X14.ConditionalFormattings();
+            ext.Append(cfContainer);
+            extList.Append(ext);
+        }
+        cfContainer.Append(x14Cf);
+    }
+
     /// <summary>
     /// Mark a worksheet as dirty. The actual save (with schema-order reorder) is
     /// deferred to <see cref="FlushDirtyParts"/> which runs in Dispose().